CONSTRUCTING DOWNSTREAM HYDRAULIC GEOMETRY RELATIONSHIPS OF THE MISSISSIPPI RIVER BASIN WITH REMOTE SENSING IMAGERY

Movement of water through river systems plays a crucial role in landscape development, in freshwater resources for human use, and in connecting the atmospheric, terrestrial, and oceanic reservoirs in the hydrologic cycle. Spatial patterns of river form and discharge are often studied using the framework of downstream hydraulic geometry (DHG), which relates downstream changes in channel flow at equivalent frequencies of discharge (Q) to variations in width (w), depth (d), and velocity (v) through three power-law equations: w=aQ^b, d=cQ^f, and v=kQ^m. Past studies have found that width is most sensitive to increasing discharge, with b-exponents of 0.5 common in alluvial rivers. Conventional methods relying on at most several hundred measurements may not capture variability in channel shape across a range of scales. In contrast, the RivWidth software tool allows continuous measurement of river width using remotely sensed imagery. In this study, we develop a continuous map of channel width for rivers wider than 100 m in the Mississippi River drainage using water classifications from the National Land Cover Dataset (derived from multi-season 30 m Landsat images). This dataset contains 1.2 x 10⁶ width measurements covering 4 x 10⁴ km of river reaches. We use discharge and channel data from U.S. Geological Survey gauging sites throughout the basin to validate the width map against in situ channel measurements taken at long-term mean flows. This dataset allows for the construction of DHG relationships for the Mississippi Basin on a range of spatial scales. Because downstream variations in river flow are unobservable with current remote sensing technology, we use drainage area as a proxy for constant-frequency discharge. Drainage area is often assumed to scale linearly with discharge and is easily calculated from digital elevation models. We link flow accumulation values from USGS HydroSHEDS 90 m DEMs to each RivWidth measurement to construct DHG relationships. We find well-developed hydraulic geometry in larger sub-basins like the Ohio River drainage (b = 0.47, R² = .74) but substantial deviation from this framework at smaller scales. This high-resolution observation of river width over large spatial extents may provide a clearer picture of the variability and limitations of DHG in describing large-scale channel form.