River Bed Elevation Variability Reflects Sediment Supply, Rather Than Peak Flows, in the Uplands of Washington State

Abstract

AbstractRiver channel beds aggrade and incise through time in response to temporal variation in the upstream supply of water and sediment. However, we lack a thorough understanding of which of these is the dominant driver of channel bed elevation change. This lack hampers flood hazard prediction, as changes to the bed elevation can either augment or reduce flood heights. Here, we explore the drivers of channel change using multidecadal time series of river bed elevation at 49 United States Geological Survey (USGS) gage sites in the uplands of Washington State, USA. We find that channel bed elevations at many of the gages change remarkably little over >80 years, while others are highly unstable. Despite regionally synchronous decadal fluctuations in flood intensity, there is a lack of regional synchrony of channel response at the decadal scale. At the monthly scale, the magnitude of antecedent high flow events between gage measurements does not predict either the direction or magnitude of shift in channel bed elevation. That variations in flood magnitude are insufficient to explain changes in bed elevation suggests that fluctuations in sediment supply, rather than variation in peak flows, are the primary driver of change to river bed elevation. In this region, channels downstream from glaciers have statistically significantly greater variability in bed elevation compared to those lacking upstream glaciers. Together, these findings suggest that aggradation and incision signals in this region predominately reflect fluctuations in sediment supply, commonly associated with glaciogenic sources, rather than response to high flow events.