128-23 Quantifying Geomorphic Change in Dryland Non-Perennial River Corridors

Abstract:

Non-perennial rivers in southwestern U.S. drylands face changing disturbance regimes due to development, diversions, and shifting flash flood frequency and intensity, but little is known about how these rivers adjust over time. Geomorphic heterogeneity, or variability of landforms, helps describe the form and functions of these rivers and their floodplains. It is generally understood that morphologic, anthropogenic, hydrologic, and biotic drivers contribute to geomorphic heterogeneity as they create, alter, and destroy landforms. While research has been conducted on the geomorphic heterogeneity of non-perennial dryland rivers, few have explored how geomorphic heterogeneity changes over time in response to static and dynamic drivers. We aim to answer questions regarding rates and characteristics of change in geomorphic heterogeneity across non-perennial river corridors. Geomorphic units were mapped using field-based transects and aerial imagery across 20 river corridor reaches from 2007 to 2023 in southern Utah and Arizona. To quantify changes to geomorphic heterogeneity, heterogeneity metrics adapted from landscape ecology—Shannon’s Diversity Index, Shannon’s Evenness Index, and Interspersion and Juxtaposition Index—were calculated for each reach over time. Reaches were then classified as sensitive, resistant, and resilient based on their heterogeneity trends. Potential drivers of geomorphic change, including drainage area, stream slope, grain size, and precipitation, were quantified and linear regression models were used to determine the relative importance of each potential driver for describing changes in geomorphic heterogeneity. Preliminary results show that most reaches experience no notable change in heterogeneity over time, with only a few reflecting sensitivity. We expect that the magnitude of change in geomorphic heterogeneity will correlate with valley morphology, land use practices, and vegetation characteristics more than precipitation. By quantifying geomorphic change at each reach and the relative importance of drivers, this work will develop general rules for how and why dryland non-perennial rivers adjust to disturbance, inform hazards in sensitive reaches, and identify rivers suitable for restoration projects.

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