Ecological and geomorphological impacts of stability restoration in urban streams

Abstract

Stream restoration projects that attempt to reduce channel incision and bank erosion by reconstructing the channel and grading and armoring stream banks (channel stability restoration projects) are common, particularly in urbanized watersheds. However, integrated assessment of changes in geomorphic processes and ecological properties within the channel and in the surrounding riparian zone induced by stability restoration has rarely been carried out across multiple restored streams. I provide such an assessment by measuring channel complexity, bed sediment dynamics, channel movement rates, riparian soil structure and function, and diatom communities in multiple restored streams located in urbanized watersheds and comparing these measurements to measurements from urban and forested reference streams.

Stability restoration appears to have reduced lateral channel migration and channel incision through channel reshaping. Patterns of bed sediment movement were altered through the effects of added channel obstructions on flow dynamics and bed sediment size distribution. Channel stability restoration did not alter channel complexity, primarily because channel complexity was not reduced by urbanization as has commonly been assumed. Restoration did not alter diatom communities either, primarily because diatom communities responded more strongly to urbanization-induced changes in water chemistry. Riparian soils were negatively impacted by stability restoration, particularly compared to riparian buffer establishment, which had mostly neutral effects on riparian soils.

Channel stability restoration can provide a minor increase in channel and bed sediment stability. However, changes in bed sediment stability were driven by in-channel restoration structures, which can be placed without grading the banks or reconstructing the channel. Riparian buffer restoration can also stabilize channels and will provide wood to channels, which can provide similar stabilization benefits as restoration structures. Restoration of channel stability using only in-channel structures and riparian vegetation planting would reduce the cost of stability restoration and reduce negative impacts to riparian soils.

Even so, effects of stability restoration were often overwhelmed by processes operating beyond the channel boundaries, suggesting that reach-scale targeting of channel instability needs to be assessed at the watershed scale and may need to be given lower priority to such restoration approaches as stormwater management, which directly address the causes of channel instability.