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Biogeomorphic Feedbacks in the Southwestern USA: Exploring the Mechanisms of Geomorphic Change and the Effectiveness of Mitigation Measures; David Dean

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Biogeomorphic Feedbacks in the Southwestern USA: Exploring the Mechanisms of Geomorphic Change and the Effectiveness of Mitigation Measures
David Dean1* and David Topping1
1U.S. Geological Survey, Southwest Biological Science Center, Grand Canyon Monitoring and Research Center, Flagstaff Arizona, USA;
Human water use, water resource development, and the proliferation of riparian plants have resulted in extensive geomorphic change to rivers worldwide. In many dryland rivers of the Southwestern U.S., hydrological change, combined with the expansion of riparian plants, have contributed to processes of channel narrowing, floodplain aggradation, and loss of fluvial habitat. Using a series of field studies, we demonstrate how sediment accumulation and vegetation proliferation have resulted in biogeomorphic feedbacks that have promoted channel narrowing and floodplain formation.
In the Little Colorado River in Arizona, water management practices, variations in climate/hydrology, and the resultant expansion of riparian vegetation have resulted in channel narrowing of up to 88%. Narrowing has occurred concurrently with increases in sinuosity and channel roughness and decreases in channel slope. These changes have created a biogeomorphic feedback by increasing flood-wave travel time and contributing to the attenuation of flood peak magnitude, thereby resulting in additional sediment accumulation. In the Rio Grande in Big Bend National Park, channel narrowing and floodplain aggradation has led to the loss of channel capacity, an increase in overbank flooding and continued floodplain accretion even though discharge has declined. In the Rio Grande and Green River, vegetation expansion onto active channel bars has resulted in bar stabilization, caused vertical aggradation of these surfaces, and has converted them to floodplains. Analyses in the Rio Grande, Little Colorado River, and Green River show that vegetation expansion into once active channel environments occurs during consecutive years of low peak flow magnitude.
An understanding of the mechanisms that have driven geomorphic changes in river channels may help to formulate effective mitigation measures. Vegetation removal can have local and reach-scale effects on channel morphology; however, the effectiveness of these actions is dependent upon many variables including the flow regime and upstream sediment supply. At larger scales, the comprehensive measurement of sediment transport, e.g., our measurement programs in the Rio Grande and Colorado, Yampa, and Green rivers, can help managers tailor upstream water releases required to maintain sufficient channel complexity or to maximize sediment export and erosion.

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