Historical Changes in Flood Power and Riparian Vegetation in Lower Harris Wash, Escalante River Basin, Utah

Abstract

This study quantifies historical changes in flood power (measured by boundary shear stress and unit stream power) and riparian vegetation in a narrow stream canyon. Analyses of historical air and ground photographs since 1922 in lower Harris Wash indicate the occurrence of active channel widening, floodplain narrowing, and an 86 percent increase in riparian vegetation growth on the canyon bottom. To quantify temporal changes in flood power at a cross-section, the WinXSPRO channel cross-section analyzer calculated stage and flood power at various return interval flood discharges for 1922 (using a ground photo) and 1998 (using field measurements). Inputs to the program included estimates of Manning's roughness coefficients and channel and floodplain width measurements for both years. Between 1922 and 1998, active-channel flood power values increased 11 to 53 percent, and floodplain flood power values decreased 44 to 97 percent. Of the floodplain power decrease, 20 to 45 percent (at a minimum) is directly attributable to increased hydraulic roughness caused by woody riparian vegetation establishment. This research suggests (1) that historical ground photos may be useful for quantifying temporal changes in flood power in relationship to vegetation, and (2) that riparian vegetation change has dramatically reduced floodplain flood power values in semiarid stream canyons during this century.[Key words: fluvial geomorphology, flood power, riparian vegetation, channel change, Utah.]