Journal of Geophysical Research: Earth Surface

Cover image for Vol. 122 Issue 8

Impact Factor: 3.412

ISI Journal Citation Reports © Ranking: 2016: 33/188 (Geosciences Multidisciplinary)

Online ISSN: 2169-9011

Associated Title(s): Journal of Geophysical Research

Tying bulk watershed properties to mountain river channel evolution

Rivers evolve over tens to thousands of years, winding and drifting and branching from single channels to braided, complex flows. Understanding exactly how and why rivers change course is a complex problem that depends on the erosion of the landscape and incorporates a range of variables, from local environmental conditions, such as the precipitation rate, to the topography, water chemistry, and the properties of the rocks in the watershed. At smaller scales, channel evolution depends on the physical properties of, and interactions between, individual grains of sediment. Correlating across this entire range of scales is incredibly difficult, but using a set of comparable observations from 83 watersheds in the U.S. Rocky Mountains, Mueller and Pitlick tie climatic, topographic, and geologic features to sediment processes and river evolution.

Using their data set, the authors found that lithology—the properties of the rocks in the watershed, and specifically the rocks' hardness, or erodibility—is the strongest measure of sediment availability. Other properties, such as watershed slope, precipitation rate, or drainage density, didn't statistically relate to changes in river sediment concentrations. Increases in sediment supply, in turn, are a strong control of river channel evolution in mountain rivers. In most mountainous rivers, the sediment grains lying along the riverbed are coarser than the sediment underneath. As the sediment concentration in the water increases, these two sets of sediment grains converge in size, reducing the protection offered by the coarse riverbed grains and enhancing sediment transport. This reduction in riverbed armoring eventually promotes the switch from single channel to braided rivers.

The authors note that there is still a lot of variability between watersheds with similar lithologies. They suggest that this variability could be attributable to things such as human land use, tectonics, glacial history, or fire activity.

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