Hydrogeomorphic and biotic drivers of instream wood differ across sub-basins of the Columbia River Basin, USA

Abstract

Instream wood promotes habitat heterogeneity through its influence on flow hydraulics and channel geomorphology. Within the Columbia River Basin, USA, wood is vital for the creation and maintenance of habitat for threatened salmonids. However, our understanding of the relative roles of the climatic, geomorphic, and ecological processes that source wood to streams is limited, and making it difficult to identify baseline predictions of instream wood and create targets for stream restoration. Here we investigate how instream wood frequency and volume differ between seven sub-basins of the Columbia River Basin and what processes shape these differences within these sub-basins. We collected data on wood volume and frequency, discharge and stream power, and riparian and watershed forest structure for use in modeling wood volume and frequency. Using random forest models, we found that mean annual precipitation, riparian tree cover, and the individual watershed were the most important predictors of wood volume and frequency. Within sub-basins, we used linear models, finding that some basins had unique predictors of wood. Discharge, watershed area or precipitation often combined with forest cover, riparian conifer and/or large tree cover in models of instream large wood volume and frequency. In many sub-basins, models showed at least one hydrologic variable, indicative of transport capacity and one ecological variable, indicative of the reach or upstream watershed’s ability to grow measurable instream wood. We conclude that basin-specific models yield important insights into the hydrologic and ecological processes that influence wood loads, creating tractable hypotheses for building predictive models of instream wood.