Braided streams in the greater Yellowstone region: linking geology, sediment supply, and channel morphodynamics
Abstract
Braided streams are uncommon in the Rocky Mountains of the Western U.S., with the exception of the Yellowstone region of northwest Wyoming. This observation provides a unique opportunity to investigate the conditions necessary for braided channel development across a large area of relatively similar hydroclimate. Here we address the relation between regional geology, sediment supply, and channel morphodynamics in braided stream systems through a combination of GIS and sediment load analysis, field studies, and flow modeling. Sediment transport data for over 80 streams and rivers in the northern Rocky Mountains show that bankfull sediment concentrations (bed and suspended load) increase as much 100-fold as basin lithology becomes dominated by softer sedimentary and volcanic rocks in the Yellowstone region. As sediment concentrations increase, stream beds become less armored, and bed load grain size coarsens. At the highest concentrations, surface, subsurface and bed load grain sizes approach unity and braided channel patterns are commonly observed. Field studies on five streams show a significant range in slope and grain size in braided reaches, but also an alternation with single-thread channels where coarse-grained tributaries tend to restrict valley width and fortify the streambed and banks. In order to quantify the influence of morphologic complexity on sediment transport, shear stress fields were derived at a range of flows along a braided reach using a 2-dimensional hydrodynamic model (FaSTMECH - USGS). For comparison, stress fields were obtained from velocity measurements in two adjacent single-thread reaches. The resultant stress distributions were coupled with a multi-fraction bed load transport model, with the output validated by measured bed load transport rates. The results show that flow in braided channels produces stress fields best described by gamma or exponential distributions, where small zones of high shear stress greatly enhance sediment transport intensity. By contrast single-thread reaches have a more uniform distribution, but in both cases modeled bed load fluxes are consistent with field measurements. High sediment supply is characteristic of braided streams in this study, where instability due to bar growth and decay gives rise to laterally dynamic channels. Yet where coarse sediment or valley constriction decreases the mobility of the channel boundary, single-thread channels are capable of maintaining sediment continuity with adjoining braided reaches.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMEP33D..01M
- Keywords:
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- 1825 HYDROLOGY / Geomorphology: fluvial;
- 1856 HYDROLOGY / River channels;
- 1862 HYDROLOGY / Sediment transport