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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 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.
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Channel Conveyance Variability can Influence Flood Risk as Much as Streamflow Variability in Western Washington State
Abstract Changes in the severity and likelihood of flooding events are typically associated with changes in the intensity and frequency of streamflows, but temporal adjustments in a river's conveyance capacity can also contribute to shifts in flood hazard. To assess the relative importance of channel conveyance to flood hazard, we compare variations in channel conveyance to variations in the flow magnitude of moderate (1.2 years) floods at 50 river gauges in western Washington State between 1930 and 2020. In unregulated rivers, moderate floods have increased across the region, but in regulated rivers this trend is suppressed and in some cases reversed. Variations in channel conveyance are ubiquitous, but the magnitude and timing of adjustments are not regionally uniform. At 40% of gages, conveyance changes steadily and gradually. More often, however, conveyance variability is nonlinear, consisting of multidecadal oscillations (36% of gages), rapid changes due to unusually large sediment‐supply events (14% of gages), and increases or decreases to conveyance following flow regulation (10% of gages). The relative importance of conveyance variability for flood risk depends on the mode of adjustment; in certain locations with historic landslides, extreme floods, and flow regulation, the influence of conveyance changes on flood risk matches or exceeds that of streamflow at the same site. Flood hazard management would benefit from incorporating historic long‐term and short‐term conveyance changes in predictions of future flood hazard variability.
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- Award ID(s):
- 1663859
- PAR ID:
- 10373031
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 58
- Issue:
- 6
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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