Influence of groundwater on distribution of dwarf wedgemussels (Alasmidonta heterodon) in the upper reaches of the Delaware River, northeastern USA

Abstract

The remaining populations of the endangered dwarf wedgemussel (DWM) (Alasmidonta heterodon) in the upper Delaware River, northeastern USA, were hypothesized to be located in areas of greater-than-normal groundwater discharge to the river. We combined physical (seepage meters, monitoring wells and piezometers), thermal (fiber-optic distributed temperature sensing, infrared, vertical bed-temperature profiling), and geophysical (electromagnetic-induction) methods at several spatial scales to characterize known DWM habitat and explore this hypothesis. Numerous springs were observed using visible and infrared imaging along the river banks at all three known DWM-populated areas, but not in adjacent areas where DWM were absent. Vertical and lateral groundwater gradients were toward the river along all three DWM-populated reaches, with median upward gradients 3 to 9 times larger than in adjacent reaches. Point-scale seepage-meter measurements indicated that upward seepage across the riverbed was faster and more consistently upward at DWM-populated areas. Discrete and areally distributed riverbed-temperature measurements indicated numerous cold areas of groundwater discharge during warm summer months; all were within areas populated by DWM. Electromagnetic-induction measurements, which may indicate riverbed geology, showed patterning but little correlation between bulk streambed electromagnetic conductivity and areal distribution of DWM. In spite of complexity introduced by hyporheic exchange, multiple lines of research provide strong evidence that DWM are located within or directly downstream of areas of substantial focused groundwater discharge to the river. Broad scale thermal-reconnaissance methods (e.g., infrared) may be useful in locating and protecting other currently unknown mussel populations.

Hydrol. Earth Syst. Sci., 20, 43234339, 2016 www.hydrol-earth-syst-sci.net/20/4323/2016/ doi:10.5194/hess-20-4323-2016 Author(s) 2016. CC Attribution 3.0 License.

Inuence of groundwater on distribution of dwarf wedgemussels (Alasmidonta heterodon) in the upper reaches of the Delaware River, northeastern USA

methods (e.g., infrared) may be useful in locating and protecting other currently unknown mussel populations.

1 Introduction

The sedimentwater interface is an important ecotone that harbors many organisms evolved to live in this dynamic environment. Areas where groundwater discharge to rivers is focused commonly are far less dynamic, particularly with regard to temperature and sediment saturation, and some organisms rely on this stability to survive (Hayashi and Rosenberry, 2002; Smith, 2005). Such may be the case for the endangered dwarf wedgemussel (Alasmidonta heterodon, family Unionidae). Formerly distributed throughout eastern North America from North Carolina to New Brunswick (Moser, 1993), this species is now endangered. It was thought to be extirpated in the upper Delaware River until populations were found along three reaches in 2000 (Cole et al., 2008). Water is diverted from three reservoirs in the upper tributaries to New York City. As a result, persistent low ows downstream from the dams during some summers may have contributed to the demise of formerly robust dwarf wedge-mussel (DWM) populations. Given the precarious status of these three DWM populations, it is imperative to understand the processes that favor their viability.

Previous studies investigated whether the riverbed in these locations would become dry during low-ow events. Although low-ow conditions dewatered much of the riverbed,

Published by Copernicus Publications on behalf of the European Geosciences Union.

Donald O. Rosenberry1, Martin A. Briggs2, Emily B. Voytek3, and John W. Lane2

1US Geological Survey, MS 413, Bldg. 53, DFC, Denver, CO 80225, USA

2US Geological Survey, 11 Sherman Place, Storrs, CT 06279, USA

3Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401, USA

Correspondence to: Donald O. Rosenberry ([email protected])

Received: 21 April 2016 Published in Hydrol. Earth Syst. Sci. Discuss.: 18 May 2016 Revised: 9 September 2016 Accepted: 26 September 2016 Published: 25 October 2016

Abstract. The remaining populations of the endangered dwarf wedgemussel (DWM) (Alasmidonta heterodon) in the upper Delaware River, northeastern USA, were hypothesized to be located in areas of greater-than-normal ground-water discharge to the river. We combined physical (seepage meters, monitoring wells and piezometers), thermal (ber-optic distributed temperature sensing, infrared, vertical bed-temperature proling), and geophysical (electromagnetic-induction) methods at several spatial scales to characterize known DWM habitat and explore this hypothesis. Numerous springs were observed using visible and infrared imaging along the river banks at all three known DWM-populated areas, but not in adjacent areas where DWM were absent. Vertical and lateral groundwater gradients were toward the river along all three DWM-populated reaches, with median upward gradients 3 to 9 times larger than in adjacent reaches. Point-scale seepage-meter measurements indicated that upward seepage across the riverbed was faster and more consistently upward at DWM-populated areas. Discrete and areally distributed riverbed-temperature measurements indicated numerous cold areas of groundwater discharge during warm summer months; all were within areas populated by DWM. Electromagnetic-induction measurements, which may indicate riverbed geology, showed patterning but little correlation between bulk streambed electromagnetic conductivity and areal distribution of DWM. In spite of complexity introduced by hyporheic exchange, multiple lines of research provide strong evidence that DWM are located within or directly downstream of areas of substantial focused groundwater discharge to the river. Broad scale thermal-reconnaissance

4324 D. O. Rosenberry et al.: Inuence of groundwater on distribution of dwarf wedgemussels

areas populated by DWM remained wetted as long as river discharge exceeded 15.8 m3 s1 at the US Geological Survey (USGS) Callicoon gage (Cole et al., 2008). Another concern was excessively warm (or cold) water temperature.Other unionid mussel species are sensitive to warm water (Galbraith et al., 2012), and DWM are particularly sensitive (H. Galbraith, personal communication, 2014). Excessive shear stress also can reduce mussel populations (Hardison and Layzer, 2001; Layzer and Madison, 1995). Shear stress during high river stage in the three remaining DWM-populated areas is smaller than the reach-averaged values (Maloney et al., 2012). Groundwater discharge may also be related to the location of mussel refugia (McRae et al., 2004), perhaps especially so in the upper Delaware River where groundwater discharge may locally diminish streambed de-watering (J. Cole, personal communication, 2013). Characterizing groundwater discharge dynamics to known patches of DWM is an important step toward a better understanding of their preferred habitat and successful management of the species.

Quantication of exchange between groundwater and surface water is particularly difcult in coarse-grained uvial settings (e.g., Gonzlez-Pinzn et al., 2015) due to spatial and temporal heterogeneity and multiple scales of ow that complicate distinction between hyporheic exchange and larger-scale groundwater discharge. Instruments are difcult to install where cobbles and boulders are present at and beneath the bed. Hydraulic gradients commonly are very small and difcult to resolve. Water chemistry is often a good method for distinguishing groundwater from surface water but, unfortunately, the chemistry of groundwater and surface water in the upper reaches of the Delaware River were found to be virtually identical.

Fortunately, use of several substantially different methods can minimize uncertainty and provide redundancy where some installations are difcult to impossible or when results based on a single method are inconclusive. The hydraulic head in water-table wells near the riverbank can be compared to river-surface elevations to evaluate the potential for lateral groundwater discharge on a reach scale. Electromagnetic-induction methods can indicate changes in streambed geology over many kilometers, particular in areas where stream water and groundwater are of similar electrical conductivity (Ong et al., 2010). These methods can be used in combination with point-scale measurements to obtain a more comprehensive, process-based understanding of DWM habitat.Point-scale physical methods, such as in-river piezometers and seepage meters (Rosenberry et al., 2008), indicate the direction and magnitude of ow across the sedimentwater interface at specic locations. Streambed vertical temperature proles can also be used to determine seepage direction and rate, and can extend point-in-time measurements of water ux to month-long time series of sub-daily ux estimates using automated analytical (e.g., Irvine et al., 2015; Gordon et al., 2012) and numerical (e.g., Koch et al., 2015; Voytek

et al., 2014) 1-D models. Other temperature-based methods can be used to separate groundwater discharge from superimposed hyporheic ow. Thermal infrared (TIR) and ber-optic distributed temperature sensing (FO-DTS) methods are used to collect large eld-of-view (100 s of m) or extensive longitudinal (km) water-temperature measurements (Hare et al., 2015). TIR does not penetrate the water surface, whereas FO-DTS measures temperature along the sedimentwater interface.

We used the above-listed methods to investigate the occurrence and distribution of groundwater discharge along three reaches of the upper Delaware River. Along each reach, which we refer to as sites, we compared results where DWM were present with results where they were absent. Specically, we pursued three main goals:

1. determine the spatial distribution of the rate and direction of water exchange across the sedimentwater interface related to the distribution of DWM populations;

2. evaluate temperature dynamics at the sedimentwater interface during warm, summer low-ow periods to investigate larger-scale groundwater discharge distributions, and determine whether areas populated by DWM may serve as cold thermal refugia;

3. investigate the geology of the riverbed and relate groundwatersurface-water exchange to potential geologic controls.

During the course of the investigation, we discovered a relatively large spring within an area populated by DWM and studied in detail the thermal inuence on adjacent and downstream water (Briggs et al., 2013). Here we expand the scope more broadly to address the three goals listed above with data collected at all three DWM-populated reaches of the Delaware River.

2 Study area

The three study sites containing DWM are within the 43 km reach of the upper Delaware River between Hancock and Callicoon, NY (Fig. 1). Prior to collection of data for this study, these sites were surveyed in 2012 by biologists familiar with DWM to determine the riverbed areas currently occupied by DWM. Each site encompassed areas where DWM were found as well as similar adjacent or nearby areas where DWM had never been found. Previous studies at these same three sites investigated minimum ows and temperature stability (Cole et al., 2008) and modeled shear stress related to occurrence of DWM (Maloney et al., 2012). Site 1 extends along the right (descending) side of a mid-channel island (Fig. 2a). Site 2 extends along a straight reach of the river where a single channel exists, and is centered above and below an ephemeral stream that enters the river on the right bank, approximately separating the known DWM area (M)

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D. O. Rosenberry et al.: Inuence of groundwater on distribution of dwarf wedgemussels 4325

New York

Pennsylvania

New Jersey

Northeast USA

Deposit, NY

0 50 100 m

(c)

30 km

Figure 2. Sites 1 (a), 2 (b) and 3 (c). Arrows indicate direction of river ow. Sensor locations pertain to seepage-meter and in-river piezometer installations.

was 45.3 m3 s1. River discharge during site visits in 2012 was close to a normally low value of about 30 m3 s1, but discharge was much larger than normal during a site visit in 2013, ranging from 100 to 320 m3 s1 (Fig. 3).

3 Methods

3.1 Geomorphic parameters

Grain-size distribution of the bed surface was determined using the Wolman (1954) pebble-count method over an approximately 100 m distance within each of the M and N reaches.River depth and ow velocity were measured at every M and N location at approximately the same time. River-surface

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Figure 1. Delaware River reach (highlighted) on the border between New York and Pennsylvania between Hancock and Callicoon.

and non-mussel area (N) (Fig. 2b). The known M area at site 3 is situated along the south side of a mid-channel island, while the N area is on the north side (Fig. 2c). At site 3, DWM were found at various times over a 10-year period along the entire reach of the channel south of the mid-channel island. However, during the 2012 eld season, DWM were found only along a 200 m reach at the downstream end of this channel (Fig. 2c). Because DWM were found upstream of the M reach prior to 2012, the upper portion of the channel was deemed inappropriate to serve as the N reach. Therefore, the N reach at site 3 was located across the mid-river island in the river channel to the left (north) of the island where current velocity was reduced. All locations are deliberately obscured to protect the endangered animals (Fig. 2).

Discharge (Q) at Callicoon, the downstream end of the river reach containing the three sites, has been measured by USGS since 1975 (USGS station number 01427510; http://waterdata.usgs.gov/nwis

Web End =http://waterdata.usgs.gov/nwis ). During 1975 through 2013, Q ranged from 4078 m3 s1 in June 2006 to 8.7 m3 s1 in

September 1997. The median Q during the period of record

4326 D. O. Rosenberry et al.: Inuence of groundwater on distribution of dwarf wedgemussels

1000

Daily Q 2013

Site visit 2013

Median daily Q

Daily Q 2012

Site visits 2012

10 1-Jan 1-Feb 1-Mar 1-Apr 1-May 1-Jun 1-Jul 1-Aug 1-Sep 1-Oct 1-Nov 1-Dec

3.2.2 Lateral groundwater discharge potential

A water-table monitoring well was installed adjacent to the right (southwest) bank of the river at each site to determine the hydraulic gradient between the water table and the river.Wells were installed using an auger to depths beneath land surface of 2.85 and 2.81 m at sites 2 and 3, respectively. At site 1, a monitoring well could not be installed because boulders in the bank were too large and densely distributed to auger a hole. However, a 0.46 m deep hole was dug by hand to below the water table at a distance of 7.1 m from the river shoreline. This allowed a single measurement of horizontal hydraulic gradient at site 1 (Fig. 2a).

Discharge of groundwater to the river was calculated using the standard Darcy equation:

Q = KiA, (1)

where Q is the volumetric seepage rate (m3 d1), K is the horizontal hydraulic conductivity of the sediment between the well and the river shoreline (m d1), i is the horizontal hydraulic gradient, which is the difference in head (m) between the water level at the monitoring well and the river divided by the distance from the monitoring well to the shoreline (m), and A is the cross-sectional area (m2) of a vertical plane at the river shoreline through which water must pass as groundwater discharges to the river.

A single-well slug test (Bouwer and Rice, 1976; Bouwer, 1989) was conducted in each monitoring well to estimate K. Hydraulic head higher than the river stage indicates ow from groundwater to the river; i is assigned a positive value for such a condition but i also can be negative if the ground-water head is lower than river stage. Gradient and, therefore, Q were determined every 20 min during July 2012 through June 2013 at sites 2 and 3 using data provided by Solinst submersible pressure transducers (Levelogger Edge, Junior and Barologger, Solinst Canada Ltd., Georgetown, Ontario, Canada) installed at xed locations in each monitoring well, and in secure locations in the riverbed (Fig. 2b, c).

3.2.3 Seepage meters

Seepage meters directly measure water ow across an approximately 0.25 m2 portion of a sediment bed in units of volume per area per time. Seepage meters modied for use in owing water (Rosenberry, 2008) were installed at ve locations along each M and N reach indicated in Fig. 2. Locations along each M and N reach were numbered 1 through 5 with numbers increasing with distance downstream. Meters also were installed within a spring area (Briggs et al., 2013) at site 2 where sediments were soft, ne-grained, and markedly colder. Locations within the spring area were numbered S1 and S3 with numbers increasing with distance downstream.Multiple measurements (n ranged from 3 to 7) were made at each location as an estimate of measurement uncertainty with

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Discharge, ms

3

100

Figure 3. Median daily river discharge (Q) based on the period of record 19752012 (USGS station number 01427510; http://waterdata.usgs.gov/nwis

Web End =http: http://waterdata.usgs.gov/nwis

Web End =//waterdata.usgs.gov/nwis ). Daily-average Q also is plotted during JuneJuly 2012 and JuneJuly 2013. Periods of site visits during22 June1 July 2012, 2030 July 2012, and 28 June2 July 2013, are shown as blue rectangles.

slope was surveyed along and beyond each M and N reach; combined with measurements of water depth, this provided a reach-averaged estimate of shear stress for each M and N reach. Shields stress, a dimensionless term that relates shear stress to the size of sediments on the bed (e.g., Bufngton and Montgomery, 1997), was calculated and compared to critical Shields stress to determine the likelihood that the sediment bed was mobilized based on water depths measured during site visits.

Samples for water-quality analysis were collected from piezometers installed at each M and N location at sites 2 and 3, from water-table monitoring wells, from several seeps along the river bank, from the large spring/seep at site 2 (Fig. 2b), and from the river at each site. Groundwater and river chemistry were found to be universally similar; therefore, these results are not discussed in further detail.

3.2 Evaluation of ow between groundwater and surface water

3.2.1 Visual and infrared observations

Discharge of groundwater to the river was visually evident at all three sites. Flowing water either discharged along the bank just above the river surface (sites 1 and 2) or was visible as it discharged rapidly enough to suspend sediment just beneath the river surface (sites 1 and 3). A handheld TIR camera (FLIR T620, FLIR Systems, Inc., Nashua, NH) was used to locate and measure surface-temperature anomalies related to cold groundwater seepage near the streambank. TIR data were used to quickly discern between actively owing seeps and other bank areas that were simply wet. TIR imagery represents only the temperature at the water or land surface; therefore, the cameras were most useful for identifying seeps at and landward of the shoreline and unmixed plumes of groundwater that reached the river surface (e.g., Hare et al., 2015). A bucket and stopwatch were used to quantify spring/seep discharge where conditions allowed.

D. O. Rosenberry et al.: Inuence of groundwater on distribution of dwarf wedgemussels 4327

Figure 4. EMI quadrature data at 33 030 Hz converted to bulk conductivity for (a) site 1, (b) site 2, and (c) site 3. Warmer colors indicate less conductive streambed material potentially correlating to coarse-grained surcial deposits and bedrock.

the understanding that measurement uncertainty may be inated if temporal variability is substantial.

3.2.4 Streambed piezometers

Similar to the riverbank monitoring wells, streambed piezometers can determine the potential for the direction of ow by measurement of hydraulic head within the streambed compared to the surface-water stage, but on a vertical axis. Streambed piezometers were installed directly adjacent to seepage meters at all M and N locations, except where installations were impossible due to buried boulders or where locations were so close together that one piezometer could represent both locations. Piezometers consisted of a stainless-steel pointed screen (30 mm diameter and 85 mm screened interval) connected to 27 mm diameter galvanized pipe. Piezometers were driven to approximately 0.5 to 0.6 m depth beneath the riverbed. Completion depth was less than 0.5 m if, after several attempts, buried cobbles or boulders prevented deeper installation. In some locations where vertical head difference was very small, the piezometer was driven to a greater depth to create a measured head difference larger than the measurement error. Insertion depths ranged from 0.42 to1.15 m. In-river piezometers can indicate rates of exchange at the sedimentwater interface if a value for K is measured or assumed. However, as the seepage meters already provided a direct measure of ow across the sedimentwater interface, vertical head gradients from the piezometers, iv, were combined with seepage rates from the seepage meters to determine a calculated value for vertical hydraulic conductivity, Kv, at each location.

3.2.5 Streambed vertical temperature proler

Surface temperature variations propagate downward into streambed sediments due to the sum of conduction and advection; if the conductive properties of the bed are measured or assumed, vertical advection can be determined using 1-D analytical or numerical models (Constantz, 2008). Thermistor dataloggers (iButton Thermochron DS1921Z, Maxim Integrated, San Jose, CA) were installed at depths ranging from 0 to 0.4 m in 14 of the piezometers to provide temperature proles with depth over time. These temperature records were collected for approximately 3 to 7 days. Strong, upward groundwater ow often reduces measurable diurnal signal penetration to less than 0.2 m (Briggs et al., 2014); therefore, at least 1 short complementary temperature proler designed specically to measure upward seepage was installed within the M zone at all three sites. These short prolers were constructed with four thermistor dataloggers (iButton Thermochron DS1922L) positioned at depths of only 0.01, 0.04,0.07, and 0.11 m beneath the riverbed. One such proler was installed at site 1 in close proximity to an observed bank-side seep, three prolers were installed at site 2 adjacent to seepage meters, and two were installed adjacent to seepage meters at the site 3 M reach (Fig. 4; locations indicated as 1-D temperature). Temperature records were collected for approximately 25 days.

Streambed-temperature time-series data were analyzed with the VFLUX program (Gordon et al., 2012) run in Matlab (Mathworks, Natick, MA). Diurnal signals were extracted from eld data using VFLUX and applied to the

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4328 D. O. Rosenberry et al.: Inuence of groundwater on distribution of dwarf wedgemussels

amplitudeattenuation analytical model (as described by Hatch et al., 2006) because this model has been shown to be reliable in determining upward ow rates (Briggs et al., 2014). Error associated with sediment-property uncertainty was determined using Monte Carlo analysis and adjusting sediment thermal properties within expected ranges (Briggs et al., 2012b). This method of analysis provides the ability to resolve temporal patterns of vertical seepage at sub-daily time steps over the period of temperature-data collection.

3.3 Temperature at the sedimentwater interface

A Sensornet Oryx (Sensornet House, Elstree, Hertfordshire, UK) ber-optic distributed-temperature-sensing system (FODTS) was deployed on the riverbed at sites 2 and 3 to collect continuous temperature data in space and time along linear cables (e.g., Selker et al., 2006). The stainless-steel-reinforced ber-optic cables were distributed across 585 m of the streambed at site 2 and across 944 m of the streambed at site 3. The deployment at site 2 (2124 July 2012) encompassed adjacent M and N reaches, while the site 3 installation (2527 July 2012) only covered the M reach due to length limitations of the cable. FO-DTS data were analyzed to identify locations of anomalously cold temperature and small thermal variance that may correspond with focused groundwater seepage to the river (e.g., Briggs et al., 2012a), and thermal refuge for the DWM.

FO-DTS data were collected at 4 and 10 min intervals and calibration for thermal drift was performed using a continuously mixed ice bath monitored dynamically by a Sensornet thermistor-type thermometer. Approximately 30 m of cable were placed in the calibration ice bath. The standard deviation of the recorded FO-DTS temperatures in the ice bath, determined to be 0.07 C, was used to estimate the precision of the FO-DTS datasets. The cable on the bed was geo-referenced by correlating survey points taken with a Nivo 5M total station (Nikon-Trimble Co., Ltd, Tokyo, Japan) with meter marks printed on the cable jacket.

In addition to the spatial coverage provided by the linear FO-DTS cables, manual point (snapshot) measurements of streambed temperature were collected at 0.05 m sediment depth using a high-precision (0.01 C) digital thermometer (Traceable Thermometer, Control Company, Friendswood, TX) at both M and N reaches of sites 2 and 3, similar to the method described by Lautz and Ribaudo (2012). Discrete bed temperatures were collected over approximately 2 h at site 2 (n = 107) on 22 July, and over 2.5 h at site 3 on 25 July

(n = 149) at geo-referenced locations. Data were laterally in

terpolated to generate areal streambed-temperature maps using the ArcMap 10 (ESRI, Redlands, CA) nearest neighbor method.

3.4 Geology of the riverbed

Bedrock and unconsolidated materials have characteristic electrical-conductivity properties that can be sensed remotely with a variety of geophysical tools. Multi-frequency electromagnetic-induction (EMI) data were used to make inferences about underlying geologic structure of the streambed; EMI has been used previously to better constrain exchange between groundwater and surface water at landscape scales (e.g., Ong et al., 2010). These data were collected at all three sites using a portable digital, multi-frequency, electromagnetic conductivity sensor (GEM-2; Geophex, Inc., Raleigh, NC) that measures the bulk apparent subsurface electrical conductivity (or magnetic susceptibility). Variance in electrical conductivity provides information about groundwater quality (e.g., salinity) or substrate properties, such as porosity. Larger conductivity values correspond to more conductive subsurface materials, such as shale bedrock or near-surface materials with a higher silt or clay fraction, whereas smaller conductivity values may indicate sandstone bedrock or coarser-grained surcial deposits.GEM-2 can be used to estimate streambed characteristics at depths up to approximately 12 m depending on streambed composition.

Multi-frequency EMI data were collected at all three sites.

A xed land location was established at each site and visited at the beginning and end of each survey to correct for instrument drift. The instrument was suspended about 1 m above the water surface using non-metallic PVC pipe secured inside an inatable raft. A kayak and drogue were used to position the raft to provide areal coverage of the riverbed. All GEM-2 land locations and surveys were geo-referenced with an on-board GPS unit.

4 Results

4.1 Geomorphic parameters

Median water depths measured at site 2 during the June 2012 eld visit were 0.58 and 0.59 m for M (DWM present) and N (DWM not present) locations, respectively. Median depths at site 3 for M and N locations were 0.41 and 0.44 m, respectively (Table 1). Median river velocities were virtually identical between M (0.18 m s1) and N (0.17 m s1) measurement locations at site 2. However, because the N locations at site 3 were in the wider and deeper channel north of the mid-channel island, median velocity at the N locations was nearly 4 times faster than at the M locations (Table 1).Only at locations M4 and M5 were current velocities at site 3 approximately the same as M-location velocities at site 2.

Reach-averaged shear stress was nearly identical at the M and N locations at site 2, primarily because the slope of the river surface (0.00037) was the same at both reaches. The M reach at site 3 also had virtually the same slope (Table 1).

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D. O. Rosenberry et al.: Inuence of groundwater on distribution of dwarf wedgemussels 4329

Table 1. Median values for parameters measured at each installation location at sites 2 and 3.

Site Location u h q l i Kv Med Med Med Med Med D50 (m s1) (m) (cm d1) (m) (m d1) u Kv h q i (N m2) (mm)

M1 0.18 0.52 0.43 0.28 0.0088 0.5 M2 0.61 2.32

Site 2 M3 0.17 0.57 0.52 0.44 0.0079 0.7 0.18 0.5 0.58 0.43 0.009 1.85 33 0.003M4 0.23 0.58 0.13 0.18 0.0114 0.1 M5 0.61 0.43

N1 0.16 0.59 0.42 0.65 0.0031 1.3N2 0.61 0.14 1.15 0.0035 0.4Site 2 N3 0.17 0.63 18.06 0.68 0.0044 40.9 0.17 1.3 0.59 0.18 0.003 1.70 4.4 0.024

N4 0.57 0.38 1.09 0.0004 9.6

N5 0.17 0.50 0.18 0.36 0.0084 0.2

Site 2 S1 0 0.27 0.56 0.60 0.0168 0.3S3 0.03 0.52 2.20

M1 0.04 0.36 3.55 0.55 0.0009 39.4M2 0.06 0.50 6.96 0.59 0.0118 5.9Site 3 M3 0.08 0.41 3.5 0.42 0.0072 4.9 0.08 12.5 0.41 3.55 0.009 1.36 43 0.002

M4 0.11 0.38 0.12

M5 0.19 0.50 26.92 1.39 0.0140 19.2

N1 0.26 0.38 0.01

N2 0.34 0.46 84.22 0.56 0.0027 313.0Site 3 N3 0.31 0.44 17.91 0.56 0.0009 202.2 0.31 202.2 0.44 0.01 0.001 4.71 55 0.005

N4 0.31 0.43 4.75 0.55 0.0119 4.0

N5 0.30 0.58 0.01

u, current velocity. h, water depth at measurement location. q, seepage ux. l, depth of in-river piezometer screen beneath riverbed. i, hydraulic gradient at each in-river piezometer. Kv, vertical hydraulic conductivity determined at each in-river piezometer. , reach-average shear stress. D50, median grain size of bed surface. , reach-average Shields stress.

The slope at the N reach at site 3 was nearly twice as large at0.00065. Therefore, shear stress at site 3, reach N, was more than double that of any of the other reaches. Shields stress (Table 1) at all reaches was well below commonly assumed critical values of 0.03 to 0.06 required for bed mobility (e.g., Shvidchenko et al., 2001).

River slope and water depths were not measured at site1. Maloney et al. (2012) indicated that water depth, current velocity, and shear stress at site 1 are similar to site 2 during river discharge (less than about 100 m3 s1). Cole et al. (2008) also indicated similar water depths between sites 1 and 2 in portions of the riverbed where DWM are known to be present.

4.2 Groundwatersurface-water exchange

4.2.1 Visual and infrared observations

Walking along the site 1 riverbank within, above, and below the reach where DWM have been identied revealed 10 bank-side seeps on both sides of the channel southwest of the mid-channel island (Fig. 2a). Small wetland areas of approximately 10 to 30 m2 areal extent also existed uphill of the seeps southwest of the channel, particularly in the area where the monitoring-well excavation was made (Fig. 2a). These wetland areas were situated 1 to 1.5 m above the river surface and between 2 and 8 m from the riverbank and were characterized by saturated soft sediments. Seeps along the left bank (island side) of the southern channel were more dis-

tinctly located. Discharge at the seep adjacent to the north-ernmost extent of the DWM area in Fig. 2a was sufcient to cause groundwater sapping, resulting in landward erosion of sediment along a 0.1 to 0.2 m vertical face at the shoreline. Bankside seeps were neither identied within 200 m upstream of the northernmost seep or downstream of the southernmost seeps in Fig. 2a, nor were any seeps identied along the north side of the island. A spring situated uphill from the river along a road cut discharged 6.4 L min1.

The spring area at site 2 (Figs. 2b and 5) included two areas approximately 0.1 m in diameter and separated by about0.5 m that discharged copious amounts of water and is described in detail in Briggs et al. (2013). The smaller spring discharged 12.9 L min1 and the larger spring discharged76.5 L min1 at a nearly constant rate. Combined, they discharged nearly 129 m3 d1. TIR imagery at site 2 indicated water issuing from these two spring discharge points had a steady temperature of 10.8 C. This cold, dense plume of un-mixed groundwater plunged into the river within 2 m of the shoreline (Fig. 5). A small volume of discharge also originated at the mouth of the ephemeral stream that was about 40 m upriver of the large seep area (Fig. 2b). No other bank-side seeps were identied along or adjacent to this study reach nor were any observed along the northern riverbank opposite the study area.

At site 3, the riverbank immediately southwest of the riverbed area where DWM have been located did not contain any obvious seeps, but the 10 to 15 m wide bench imme-

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4330 D. O. Rosenberry et al.: Inuence of groundwater on distribution of dwarf wedgemussels

Figure 5. (a) Photograph of riverbank at site 2 with red rectangle indicating area of infrared image (b). Color infrared image with blue area showing colder groundwater entering the river. Color scale indicates temperature, in C.

diately adjacent to the shoreline was soft and wet in areas. Two seeps were identied at the shoreline next to the mid-channel island, approximately equidistant from the two west-ernmost M locations (Fig. 2). These seeps discharged water both above and below the shoreline and suspended sand where the discharge point was submerged. Five other colder seeps were located upriver along the right bank (south side) of the channel. The discharge point at all of the right-bank seeps was 0.1 to 0.2 m above the river surface. Although difcult to measure, several of the seeps were discharging at approximately 0.5 to 2 L min1. Seeps along the mid-channel island at site 3 were universally warmer (22.625.7 C) than those observed at other sites. Seeps on the south side of the channel at site 3 were not measured but they were noticeably colder than the island seeps and were similar in temperature to the site 2 seeps. Temperature in the monitoring wells at sites 2 and 3 averaged 11.9 and 11.4 C, respectively, during the period when spring and seep temperatures were measured.

4.2.2 Lateral groundwater discharge potential

The single measurement of hydraulic gradient (i) at site 1 during the afternoon of 26 July 2012 resulted in a value of 0.17, indicating a large potential for groundwater to discharge to the river. The stabilized water level in the exca-

vation was 1.22 m above river stage. With no value for K for this location, no attempt was made to determine the rate of groundwater discharge to the river along this reach. This measurement was made during a prolonged period of relatively steady river discharge that began on 24 June and likely represents largely steady-state conditions.

The median value for i at site 2, determined over nearly a year, was 0.08 (Fig. 6a), also quite large for sandy sediments.Values were typically close to 0.07 during summer months and increased to about 0.10 starting in November and continuing until mid-March. The smallest gradients, other than during gradient reversals, occurred during mid-May to mid-June when river stage rose to a greater extent than groundwater. K based on analysis of slug-test data from the site 2 monitoring well was 6.3 102 m d1, indicative of silty sand. The esti

mate of the thickness of quaternary alluvium and underlying transmissive fractured sandstone was based on Braun (2011) and Martin et al. (1985). Assuming the transmissive sediment between the riverbed and the underlying bedrock is 10 to 20 m thick, and assuming that all of the horizontal ow of groundwater to the river occurs through this 10 to 20 m thick vertical cross section of sand and fractured shallow sandstone before discharging to the river, groundwater discharge to the river at site 2 would be 0.04 to 0.07 L min1 per meter of river reach. For the entire site 2 reach, groundwater discharge would be about 10 to 20 m3 d1. This is likely an underestimate as slug tests generally yield smaller values of K than more scale-appropriate methods (e.g., Schulze-Makuch et al., 1999; Rovey II and Cherkauer, 1995).

The median value for hydraulic gradient based on the monitoring well at site 3 was 0.05 (Fig. 6c). Other than during high-discharge events, the median value was remarkably stable during the period of record. The gradual decrease in hydraulic gradient from late July until early September 2012, is likely a return to hydrostatic conditions following well installation and indicative of the low K of the sediments at the well. More than a month was required for the water level inside the well to stabilize. Slug tests indicate that K is 8.5 105 m d1, 3 orders of magnitude smaller than at

site 2. Making the same assumption that groundwater discharge to the river occurs through a vertical plane at the shoreline that is 10 to 20 m thick, groundwater discharge at site 3 would be on the order of 3 to 6 105 L min1 per m

of river reach, or 0.03 to 0.06 m3 day1 for the entire site 3 reach. Given the numerous springs in this area, this slow, diffuse groundwater discharge clearly is augmented by focused groundwater discharge through preferential ow paths.

Relative river stage and water level in the adjacent ground-water monitoring well are plotted in Fig. 6 for sites 2 and 3.Values are adjusted so river stage approximately equals the water depth at each in-river pressure transducer. As the river stage rose, the shoreline moved laterally and the distance between the shoreline and the monitoring well decreased. Calculations of i incorporated a linear interpolation of reduced horizontal distance with increasing river stage, with a min-

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D. O. Rosenberry et al.: Inuence of groundwater on distribution of dwarf wedgemussels 4331

Figure 7. Median values of seepage ux. Error bars indicate maximum and minimum measured values. Median value for site 3 N2 is 84 cm d1.

4.2.3 Flow measured at seepage meters

Seepage was generally small at all but a few M and N measurement locations. Median values of seepage were upward at 8 of 10 M locations and at 6 of 10 N locations. Both M reaches had positive (upward) median values (0.43 and3.55 cm d1) and both were larger than median values for the N reaches (0.18 and 0.01 cm d1) (Table 1). The only reach

where seepage was substantially upward was the M reach at site 3, where upward seepage at 4 of 5 locations was greater than 3 cm d1. Seepage was more variable at N reaches than at M reaches, particularly so if the site 3 M5 value is excluded (Fig. 7). Seepage at that location was much more variable due to increased turbulence where two channels merge into a single channel. The large variability at the N reach of site 3 (Table 1) was undoubtedly due to the larger current velocity, shear stress, and greater hyporheic exchange there.

Seepage at locations S1 and S3 at site 2 was only slightly to moderately faster relative to seepage measured at nearby M locations (Table 1). Although larger rates of seepage were expected within this cold-water spring area, detailed temperature measurements indicated that most of the seepage, and the source of the cold water at the streambed, originated land-ward of the shoreline (Briggs et al., 2013).

4.2.4 Streambed piezometers

Vertical hydraulic gradients (iv) measured at in-river piezometers were generally small and indicated the potential for upward ow at all M locations and at 6 of 8 N locations.Where iv was negative (site 2 N4, site 3 N4), indicating the potential for downward ow, q determined with seepage meters was also negative, indicating downward ow (Table 1).The median of M-reach iv measurements was 0.009 at both

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Figure 6. River stage, water-table elevation, and hydraulic gradient at sites 2 and 3. Legend in panel (b) also applies to panels (a) and(c) except site 3 data are from the WT2 monitoring well. (a) Site 2, July 2012 through June 2013; (b) 20 min data from site 2 showing gradient reversal on 19 September 2012; (c) site 3, July 2012 through June 2013.

imum horizontal distance of 0.9 m occurring at a maximum relative river stage of 3.53 m at site 2, and a minimum horizontal distance of 0.7 m occurring at a maximum relative river stage of 2.93 m at site 3. At both sites, maximum river stage occurred on 19 September 2012 (Fig. 6), indicating the high-stage river shoreline did not quite reach the locations of the monitoring wells.

During the 1-year period from July 2012 through June 2013, the hydraulic gradient at both sites 2 and 3 reversed and became negative 7 times in response to a rising river stage that preceded and exceeded a corresponding rise in the water table at the monitoring wells. This effect is displayed in Fig. 6b for the largest rise in river stage at site 2 on 19 September. The rapid increase in river stage from 03:00 to 06:00 LT was substantially larger than the responding increase in head at well WT1, resulting in a reversal of hydraulic gradient that exceeded 0.7 for a short time.

The gradient reversal was short lived, however, lasting only about 10 h. An upward hydraulic gradient was re-established at about 13:00 LT. Furthermore, even though river stage at 22:00 LT was still 0.75 m higher than it was prior to the high-stage event, hydraulic gradient had already returned to the pre-event value of 0.04.

4332 D. O. Rosenberry et al.: Inuence of groundwater on distribution of dwarf wedgemussels

M reaches. Median values at N reaches were 0.003 and 0.001 for sites 2 and 3, respectively. The piezometer installed at S1 in the spring area at site 2 (Figs. 2b and 5a) indicated a relatively large iv of 0.017.

Calculated Kv at about half of the measured locations was smaller than expected for a gravel- to cobble-bedded river, indicating that ner-grained sediments were present between the bed surface and the well screen at some locations. Values for Kv ranged from 0.1 to 39 m d1 at M-reach locations and from 0.2 to 313 m d1 at N-reach locations. The two largest Kv values were at N2 and N3 at site 3, where the current is faster and cobbles are larger. These values, both larger than 100 m d1, are indicative of coarse sand or gravel. Median values of Kv determined at M reaches were 0.5 and 12.5 m d1 at sites 2 and 3, respectively. Median values of Kv at corresponding N reaches were 1.3 and 202 m d1. The value for Kv at location S1 in the spring area was only 0.3 m d1 (Table 1), indicative of silty sand, such as was observed on the bed in this area.

4.2.5 Streambed vertical temperature prolers

Vertical seepage rates determined with VFLUX from the thermal records collected in piezometers varied substantially depending on which pair of thermometers was used to calculate q (Table 2). At site 2, thermal results indicated that rapid downward seepage near the surface of the riverbed decreased with depth at locations M3 and N3, whereas seepage-meter results indicated small (M3) to moderate (N3) upward seepage. Results at site 3 N3 were similar to those at site 2 N3 but with smaller values. Only at site 3 M5 did the thermal proler records collected in piezometers and seepage-meter results indicate seepage in generally the same direction. Results from thermal models and seepage-meter values were similar at M2 and also at M5 if the 3050 paired thermometers were used (Table 2).

Seepage determined with temperature data from the shallow prolers designed to capture upward ow averaged 16 cm d1 at site 1 (installed near a visible bankside seep)

and steadily decreased over the recording period (Fig. 8).

Shallow proler seepage at site 2 near the main spring (2 to 3 m from the S1 and S3 seepage meters) averaged 27 cm d1 and varied from 12 to 39 cm d1 (Fig. 8, uppermost red line).The remaining proler data from two of the three locations at site 2, and both locations at site 3, showed fairly consistent uid ux that can be described as circumneutral as they ranged within the expected error bounds of 10 cm d1 asso

ciated with this method in this coarse-grained setting (Fig. 8).

All four of these circumneutral plots show two downward spikes in seepage, the latter coincident with an upward spike in river stage and discharge measured at the USGS gage in Callicoon.

Slow seepage-ux estimates in the range of 10 cm d1

from prolers at sites 2 and 3 generally correspond with nearby seepage-meter rates ranging from 0.5 to 7.0 cm d1.

Figure 8. Seepage rates determined with VFLUX and Delaware River discharge determined over 25-day period from 28 June23 July.

Table 2. Comparison of time-averaged VFLUX seepage values and median seepage-meter values at select locations.

Site Location Interval VFLUX Seepage meter (cm) (cm d1) (cm d1)

2 M3 010 69.2 0.5

2 M3 1020 12.1 0.5

2 M3 2040 9.9 0.5

2 N3 010 32.2 18.1

2 N3 1020 18.6 18.1

2 N3 2030 10.2 18.1

2 N3 3050 5.0 18.1

3 M2 010 4.4 73 M5 1020 11.1 26.9

3 M5 2030 0.5 26.93 M5 3050 32.5 26.93 N3 020 19.7 17.9

3 N3 20-30 6.9 17.9

3 N3 3050 4.4 17.9

The 27 cm d1 value from the proler installed near the spring area at site 2 was substantially larger than values of 0.56 and 2.20 cm d1 measured at the S1 and S3 seepage meters. The proler value was similar to seepage determined at four high-resolution temperature sensors installed in the spring area for a related study that averaged from 12 to 35 cm d1 (Briggs et al., 2013).

4.3 Temperature at the sedimentwater interface

Average FO-DTS temperatures collected over 4 days at site 2 ranged from 14.0 to 22.5 C (Fig. 9). A slightly colder zone was detected along a 115 m length of cable located closer to shore along much of the M reach and into the southern portion of the N reach. Discharge of cold groundwater should

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D. O. Rosenberry et al.: Inuence of groundwater on distribution of dwarf wedgemussels 4333

Figure 9. Riverbed temperatures indicated by snapshot thermal surveys (shaded riverbed areas) and FO-DTS at site 2 (panels a, b) and site 3 (panels (c), (d)). Colored circles in panels (a) and (c) indicate temperature and sizes of circles in panels (b) and (d) indicate temperature standard deviation during 4- and 2-day cable deployments at site 2 and 3, respectively.

result in decreased variance but temperature variance along the site 2 FO-DTS cable is actually largest for the colder areas, except at the bankside seep depicted in Fig. 5 (Fig. 9b). The larger variance is likely due to the nearshore cable along the cold reach being situated in shallow, clear water that often results in solar heating of the interface and cables (Neil-son et al., 2010). This is supported by data from the cable located further from shore that shows reduced thermal variance. Average FO-DTS data collected over 2 days at the site3 M reach have a much tighter range of 22.6 to 23.3 C. Temperature variance (Fig. 9d) is relatively large throughout the shallow-water area south of the island point bar, but is greatly reduced where the cable passes through stronger current from the channel that originates on the north side of the mid-river island.

Temperatures measured with the snapshot streambed thermal surveys at sites 2 and 3 are generally similar to patterns shown in the FO-DTS data. However, the snapshot data indicated several discrete cold zones near the island at site 3 that were missed with nearby FO-DTS cables. Discrete cold patches were found at sites 2 and 3 along the M zones but not in the N zones (Fig. 9). The cold anomalies make up a rel-

atively small percentage of the overall surveyed area at both

M reaches. The largest cold anomaly is located at the site 2 spring area and indicates a plunging plume of cold water, as discussed earlier. The areal extent of this anomaly is approximately twice as large as the plume footprint measured within the water column, likely indicating an inuence from more diffuse groundwater upwelling through the streambed, as detailed in Briggs et al. (2013). Cold riverbed areas were better detected with the discrete snapshot method than with the continuous FO-DTS method, likely because the snapshot measurements were made at 0.05 m depth and the ber-optic cable was resting on top of the bed and inuenced to a greater extent by surface-water temperatures. The snapshot method also provided better lateral distribution of data collection.

4.4 Geology of the riverbed

Consistent spatial patterns of streambed electrical conductivity were observed in multiple adjacent and overlapping EMI lines, but there was no apparent relation between riverbed electrical conductivity and occurrence of DWM (Fig. 4). For example, DWM areas at sites 1 and 3 are located above more

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4334 D. O. Rosenberry et al.: Inuence of groundwater on distribution of dwarf wedgemussels

conductive material, whereas corresponding N reaches are generally less conductive. Conversely, DWM at site 2 are found over the least-conductive material, whereas the opposite side of the river and N reach are both more conductive.

5 Discussion

Some of the individual methods for characterizing rates and spatial distribution of groundwater discharge produced inconclusive results, indicative of the difculty presented by such a challenging setting. Collectively, however, they lead to the conclusion that groundwater discharge is related to occurrence and distribution of DWM in the upper Delaware River.Listed from strongest to weakest, the evidence stacks up as follows:

1. Easily visible seeps and springs were present at or just upriver of all three M reaches but not at the N reaches.

2. Large lateral hydraulic gradients toward the river indicate the potential for substantial groundwater discharge at all three sites.

3. Upward seepage through the riverbed measured with seepage meters was much faster and more consistently upward at reaches populated by mussels.

4. Median upward vertical hydraulic gradients were 39 times larger at M reaches than at N reaches.

5. Seepage based on vertical temperature proles measured with two different methods of instrumentation was upward, circumneutral, and downward, at 2, 5, and 1 of 8 M-reach locations, respectively, whereas temperature-prole-based seepage was downward at all three N-reach locations.

6. Riverbed temperature based on FO-DTS and snapshot streambed thermal surveys was slightly colder in the M reaches than in the N reaches; bed temperature was particularly cold in discrete patches that were better captured with the bed temperature snapshot surveys.

Combined results from the rst four methods, in particular, provide compelling evidence that groundwater discharge is substantial in areas populated by DWM. Somewhat surprisingly, neither geophysical nor chemical methods were related to presence or absence of DWM. Although patterns were evident, geophysical data showed no clear correlation with M or N reaches. Chemistry of groundwater, water removed from in-river piezometers, and surface water was virtually identical, rendering chemistry, often a good indicator of water source, of little use to distinguish M from N reaches.

It is also clear that groundwater discharge is not evenly or universally distributed across the M or N areas. Hyporheic exchange superimposed on broader-scale groundwater discharge exerts a highly complex ow path distribution that

results in variable rates of upward, largely horizontal, and downward seepage across the riverbed. This local-scale variability in seepage direction and rate did not appear to be related to locations of individual DWM. Three pairs of seepage meters and streambed piezometers were installed nearby, and within 1 m of an individual DWM, without evidence of anomalously strong seepage at specic DWM locations.These three paired observations, along with substantial heterogeneity in seepage rate and direction within each M reach, indicate that DWM do not require focused groundwater discharge precisely where they are located, but instead rely on the existence of substantial groundwater discharge within or just upstream of their populated area.

Other studies that have investigated the effect of ground-water discharge on benthic invertebrates have yielded mixed results. One study indicated a direct correlation between rate of groundwater discharge and abundance and taxonomic richness (Hunt et al., 2006), while another showed little correlation (Schmidt et al., 2007). Few studies have related groundwater discharge with mussel abundance and species richness. A study conducted in a river with similarly coarse sediment indicated a relation between mussel population density and upward seepage rate (Klos et al., 2015), but upward seepage in that setting was primarily driven by hyporheic exchange. The net upward seepage at DWM sites in the Delaware River, although clearly inuenced by hyporheic exchange, is primarily the result of area-wide groundwater discharge as evidenced by substantially faster reach-averaged upward seepage and also colder water along M reaches relative to N reaches.

Obtaining direct measurements of groundwater discharge is difcult in settings such as the upper Delaware River where large boulders up to 1 m or more in diameter are common.Distinguishing hyporheic exchange from groundwater discharge in coarse-grained uvial settings can also be challenging (e.g., Gonzlez-Pinzn et al., 2015; Menci et al., 2014; Ward et al., 2013; Bhaskar et al., 2012), hence the multiple lines of evidence pursued for this study. Therefore, few studies of exchange between groundwater and surface water have been successfully conducted in such coarse-grained sediments. Compared to those that have (e.g., Rosenberry et al., 2012; Fritz et al., 2009; Klos et al., 2015), values for point measurements of seepage exchange at these three sites on the Delaware River were not particularly large. This indicates that hyporheic exchange is perhaps smaller than would be expected along M reaches, given the coarseness of the bed.And, just as was inferred regarding smaller-than-expected Kv values, large horizontal hydraulic gradients adjacent to the river at all three sites would indicate larger amounts of groundwater discharge to the river than were measured, implying the presence of substantially ner-grained sediment beneath the bed surface that is likely leading to focused rapid discharge in isolated locations while at the same time limiting diffuse ow. Flow strong enough to suspend sediments in the water column was observed at many of the bankside

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D. O. Rosenberry et al.: Inuence of groundwater on distribution of dwarf wedgemussels 4335

seeps. Most of the spring discharge at site 2 was focused at two locations above river stage but it clearly was sufciently rapid to suspend sediments had it been submerged. Discharge through the submerged portion of the spring area at site 2 was also heterogeneous; cold water existed throughout the spring-area sediments but ne-grained sediments were not suspended and seepage rates at S1 and S3 were not remarkably large. Fine- to medium-grained silt was attached to many of the M-reach piezometers upon removal, but no silt was observed on removal of any of the N-reach piezometers.Silts are generally more electrically conductive than gravel and cobbles, but the EMI data showed M reaches were located over a mix of sediment types. The depth-integrated ( 012 m) data presented here may not capture a shallow

layer of nes. The multi-frequency GEM2 tool can be used at higher frequencies for shallow depth-specic investigation (Briggs et al., 2016), but the use of this higher-frequency data was complicated by the variable depth of surface water, which strongly inuences the signals.

Cold-water anomalies were detected along all M reaches, but never along an N reach. At site 2, mussel-location data from 2010 and 2012, in particular, indicated a strong clustering of animals directly adjacent to and downstream from the main spring described here and by Briggs et al. (2013)(J. Cole, unpublished data). DWM indeed may be present in these areas due to relatively stable and cold groundwater discharge that serves as a refuge for these animals during periods of lowest river stage. Additionally, mussel surveys have only been taken at these locations during summer months; groundwater discharge also may offer benets for mussel survival during cold winter extremes that are not apparent based on these data collected during the summer.

Data indicating ow in opposite directions across the riverbed are initially puzzling (Table 2). Hyporheic ow paths in substantially heterogeneous and highly transmissive sediment, a common situation in a cobble-bed river, are predominantly horizontal with small upward and downward ow components. Seepage meters quantify the upward or downward component of ow across the sedimentwater interface whether the ow is vertical or largely horizontal. Because piezometers and vertical temperature prolers are installed vertically, thermally derived interpretations of seepage assume vertical ow through the sediments, often a poor assumption in hyporheic settings. It is not uncommon for seepage meters to indicate ow in one direction while hydraulic gradients indicate the opposite (Rosenberry and Pitlick, 2009; Rosenberry et al., 2012; Angermann et al., 2012; Kser et al., 2009). Locations with discordant data are indicative of ow across the sedimentwater interface that was largely driven by hyporheic processes, which are superimposed on larger-scale groundwater-discharge patterns (Rosenberry et al., 2012). Hyporheic ow appeared to dominate exchange at the site 3 N reach. Furthermore, substantial changes in the vertical component of hyporheic ow were indicated at most of the locations where temperature was mea-

sured at multiple depths in the riverbed (Table 2), also indicative of hyporheic exchange that is reduced or transitions to horizontal ow with increasing sediment depth (e.g., Briggs et al., 2012b).

5.1 Methods comparison

The rst four methods listed at the beginning of the discussion section all provide strong evidence for groundwater discharge to the river, although the scale of those measurements was not the same. Large lateral hydraulic gradients based on data from bankside monitoring wells consistently indicated potential for substantial groundwater discharge at all three study sites on a site-wide scale, but those results could not distinguish between groundwater discharge at M vs. N reaches. The other three methods were more local in scale, which allowed comparisons of M and N reaches.Direct observation of seeps and springs at M but not at N reaches, faster upward seepage at M than N reaches, and larger upward vertical hydraulic gradients at M not N reaches all indicate greater groundwater discharge in and near areas populated by DWM. However, for in-river seepage and hydraulic-gradient measurements, these conclusions can only be reached when data are aggregated within each M or N reach. Otherwise, local-scale hyporheic exchange greatly confounds the interpretation.

Hyporheic exchange also made it difcult to obtain clear interpretations from temperature-based data. Because shallow hyporheic ow paths in coarse-grained sediments are primarily horizontal and temperature-prole methods assume primarily vertical ow, it should not be surprising that data might be difcult to interpret. Other studies in ner sand-bed streams have obtained conclusive and consistent results using these methods (Rosenberry et al., 2016; Hatch et al., 2010). Excluding the questionable temperature-based data in Table 2, when all other location-specic data are aggregated over an entire M or N reach, the conclusion is consistent; greater groundwater discharge occurs at M than at N reaches.

Given the substantial hyporheic exchange that results in upward ow across the bed with basically the same temperature as the river water, it also is not surprising that the two thermal-reconnaissance methods would not show a strong difference between M and N reaches. However, the manually measured snapshot temperature survey still identied colder areas of the sediment bed, but in M reaches only. The efcacy of the snapshot method was a pleasant surprise, likely because measurements were made at 0.05 m sediment depth and better indicated the temperature of discharging ground-water.

Only the methods based on geophysics and chemistry provided data of little value. As suggested previously, geologic controls on distribution of seepage may have been of a scale that was impossible to resolve with these geophysical tools.Regarding chemistry, it may be that groundwater ow paths were neither sufciently long nor groundwater sufciently

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4336 D. O. Rosenberry et al.: Inuence of groundwater on distribution of dwarf wedgemussels

old, in this headwaters area for groundwater chemistry to have developed a water chemistry distinguishable from river water.

5.2 Seepage at study sites relative to kilometer-scale values

Substantial groundwater discharge clearly occurs at areas populated by DWM, and no areas of focused discharge were identied immediately upstream or downstream of these three DWM-populated areas. However, is this prodigious discharge greater than what is typical along the upper reaches of the Delaware River? Fortunately, river discharge can be compared between two USGS gaging stations: Lordville (USGS station number 01427207; http://wterdata.usgs.gov/nwis

Web End =http://wterdata.usgs.gov/ http://wterdata.usgs.gov/nwis

Web End =nwis ) and Callicoon. Several streams enter the river between these two gaging stations, most notably Little Equinunk and Basket Creeks, but these streams are largely dry following prolonged dry periods. During 2013, the average gain in measured river ow between Lordville and the downstream Callicoon stations was 5.9 m3 s1. During October, the month with the smallest average river ow, the gain between the two gaging stations was only 2.1 m3 s1. Divided by the 29 km distance between the two gaging stations, and assuming that stream inputs during this low-ow period were minimal, perhaps contributing half of the increase in river discharge, this equates to an average increase in discharge of 2.2 L min1 per m of river reach. Assuming that discharge of groundwater occurs equally on both sides of the river, this equates to an average rate of groundwater discharge of 1.1 L min1 per meter of river along each bank. This value is substantially smaller than most of the point-discharge values that were measured in the various seeps and springs identied along each of these study sites where DWM have been found (Table 3). Furthermore, only the seeps at and slightly above the river bank were identied either visually or with FLIR data. Based on colder locations measured along M reaches with the proler data, it is likely that other seeps also were present but they were not observed because they were submerged. Therefore, although groundwater does contribute water to other reaches of the upper Delaware River, the rate of discharge is substantially greater within areas populated by DWM. The link between groundwater discharge and DWM preference for these areas remains unknown, warranting further research. A recent study by Galbraith et al. (2015) indicates that DWM may be less mobile during dewatering caused by reduced river ow than other mussel species. Groundwater discharge may offset the effects of dewatering of the riverbed caused by rapid decreases in river stage.

6 Conclusions

Alasmidonta heterodon (dwarf wedgemussels) were located within or slightly downriver from reaches where a prodi-

Table 3. Rates of measured or calculated groundwater discharge. [Delta1]Q is the difference in river ow between two USGS gaging stations.

Site Measurement Measurement Seepage rate , type scale L min1

1 spring point 6.42 well site reach 0.04 to 0.07 2 spring point 12.92 spring point 76.53 well site reach 36 10

5

3 spring point 0.5 to 2 13 [Delta1]Q 29 km reach 1.1

Reach-scale seepage rate is per meter of river reach per single side of river.

gious amount of groundwater discharge was observed. Discrete, anomalously cold riverbed areas were detected in all DWM reaches, but never in the reaches where DWM were not detected. Measured discharges from individual seeps and springs ranged from 0.5 to 77 L min1. Discharge also occurred from numerous other visible seeps that were not measurable because it occurred right at the bank or in river water that was less than 5 cm deep. Horizontal hydraulic gradients measured at water-table wells installed within 12 m of the river were large and indicated ow from groundwater to the river at all three study sites. Although gradient at site 1 was measured only once, gradients indicating ow toward the river at sites 2 and 3 persisted year-round except for brief periods when they reversed in response to abrupt river-stage rise following large rains or snowmelt. Measurements of groundwatersurface-water exchange at specic points on the riverbed indicated that seepage was upward across the sedimentwater interface at 80 percent of DWM locations and 60 percent of non-DWM locations. Median values of seepage along DWM reaches were 0.4 and 3.5 cm d1; median values of seepage at non-DWM reaches were 0.2 and

0.01 cm d1. Vertical hydraulic gradients indicated upward ow at all locations in DWM reaches and median values were 3 to 9 times larger than at non-DWM reaches. Large rates of hyporheic exchange in places complicated the distinction between groundwater discharge at DWM vs. non-DWM reaches. Streambed-temperature-based seepage measurements guided by thermal surveys (e.g., at cold zones) consistently indicated moderate groundwater upwelling to the river, conrming these as zones of rapid upward seepage.Geology beneath the riverbed, as evaluated by bulk electrical conductivity, was variable at all three study sites, but geologic variability did not appear to be correlated with distribution of DWM.

In conclusion, the collective lines of evidence indicate that DWM are situated in or directly downstream of areas of substantial groundwater discharge to the river. The work presented here and in Briggs et al. (2013) may be the rst

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D. O. Rosenberry et al.: Inuence of groundwater on distribution of dwarf wedgemussels 4337

to demonstrate the importance of groundwater discharge to unionid species. Additional work is needed to better understand the linkages between groundwater discharge and presence of DWM as well as geological controls that focus groundwater discharge in these areas.

7 Data availability

Data on geomorphic parameters and groundwatersurface-water exchange are available upon request to Donald Rosen-berry. Temperature data, seepage rates determined from measurements of temperature, and geophysical data are available upon request to Martin Briggs.

Acknowledgements. We thank Jeffrey Cole for advice and instructions related to river and riverbed logistics, and Heather Galbraith and Carrie Blakesley for mussel identication and location, all from the USGS Northern Appalachian Research Branch. Don Hamilton from the National Park Service Upper Delaware Recreational and Scenic River, and Joseph Markos, Richeld, MN, are thanked for their eld assistance and logistical support. Jason Halms (University of Colorado-Boulder) exceptional support before, during, and following eld work is greatly appreciated. This work was funded by the US Fish and Wildlife Service. Use of trade names is for identication purposes only and does not constitute endorsement by the US Geological Survey.

Edited by: C. StammReviewed by: two anonymous referees

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