Glaciogenic sediment implicated in river bed elevation instability across the uplands of Washington State, USA
Abstract
River 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 whether water or sediment 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 multi-decadal time series of river-bed elevation at 49 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. While there are regional, decadal fluctuations in flood intensity, there is a lack of regional, decadal synchrony of channel response. At the monthly scale, the antecedent high flow between channel measurements does not predict either the direction or magnitude of shift in channel-bed elevation. That neither decadal- or event-scale variations in flood magnitude explain changes in bed elevation suggests temporal fluctuation in sediment supply, rather than variation in peak flows, is the primary driver of change to river bed elevation. For more direct evidence that sediment supply drives channel change, we investigate the relationship between basin characteristics and channel bed elevation variability. We find statistically significantly greater variability in bed elevation in channels downstream from glaciers compared to those lacking upstream glaciers. This finding is consistent with previous work in the region highlighting channel change in response to glacial fluctuations and proglacial mass wasting events. Together, our findings suggest that aggradation and incision signals in this region predominately reflect fluctuations in sediment supply, rather than response to high flow events, and that only a subset of watersheds experience substantial variations in that supply at the decadal scale.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMEP51A..02P
- Keywords:
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- 1821 Floods;
- HYDROLOGY;
- 1824 Geomorphology: general;
- HYDROLOGY;
- 1862 Sediment transport;
- HYDROLOGY