Partitioning a mass-balance sediment budget by sand-size classes to understand where, when, and why channel change occurs
Abstract
The forms of channels and floodplains are largely determined by the stream-flow regime and the watershed sediment supply. When the supply of water and sediment to a river change, the river may respond by accumulating or evacuating sediment that in turn changes the attributes and availability of aquatic habitat. Continuous measurements of sediment transport at reach-bracketing gaging stations allow for the construction of mass-balance sediment budgets that identify periods of sediment loss or accumulation; however, such measurements cannot identify where channel change will occur. Thus, it is important to link reach-scale changes in sediment mass balance to field measurements of channel change. In this study, we use a continuous mass-balance sediment budget and geomorphic analysis of channel change to evaluate the links among sediment flux, streamflow, and changes in channel morphology on the lower Yampa River, Colorado. Flux-based measurements showed that the efflux of sand exceeded the influx in every year since 2012, resulting in a sand deficit of 470,000 ± 320,000 metric tons, which corresponds to between 0.04-0.20 m of erosion averaged over a channel length of 12.5 km. We showed that in-channel alluvial deposits have different grain size characteristics, and that to predict changes in habitat availability, we need to know which grain sizes are being accumulated or evacuated. When the mass of erosion was partitioned by sand size classes, fine, medium, coarse, and very coarse sand was found to have been continuously in deficit since 2012, but very fine sand had accumulated in 2014 and since 2016. The evacuation of fine and medium sand occurred along the channel boundary during the annual snowmelt runoff and very fine sand accumulated within vegetated islands when the annual flood exceeded the 2-year recurrence interval. Coarse and very coarse sand was evacuated over the entire hydrograph and could not be linked to a net change in storage within an alluvial surface. This study demonstrates that it is possible to explicitly link changes in a sediment mass balance to changes in channel form and habitat when the mass-balance sediment budget is partitioned by grain size classes and we know where those grain sizes are stored within the channel. This allows us to understand when, where, and why channel change occurs.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMEP34A..03L
- Keywords:
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- 0442 Estuarine and nearshore processes;
- BIOGEOSCIENCES;
- 0481 Restoration;
- BIOGEOSCIENCES;
- 1813 Eco-hydrology;
- HYDROLOGY;
- 1825 Geomorphology: fluvial;
- HYDROLOGY