A RIVER RUNS THROUGH IT: MODELING AND MAPPING FLOOD INUNDATION IN YOSEMITE VALLEY, YOSEMITE NATIONAL PARK (USA)

Twelve flood events of the Merced River have inundated Yosemite Valley since 1916. These floods transpired from episodes of warm winter precipitation onto snow pack. With projected increases of “pineapple express” storm events in the Sierra Nevada and other Pacific Coast mountain ranges, knowledge of landscape modification and the surface extent of historic flood inundation in Yosemite Valley are necessary for determining the increasing impacts from future floods. The spatial modeling and mapping of the historic and potential alluvial flood inundation of Yosemite Valley was performed with recently acquired LiDAR data. A Relative Elevation Model (REM) was employed to modify LiDAR-derived digital elevation data to a constantly changing datum – the water surface elevation of the Merced River. This spatial modeling technique permitted detailed visualization and analysis of in-channel and floodplain geomorphologic features of changing elevations to the changing river water surface elevations. Initial results have constrained the river stage heights at which key in-channel and floodplain landforms activate and the lateral extent that proximal floodplains inundate. The activation of in-channel and floodplain landforms (e.g. side-channels, meander cut-offs, and bank heights) and floodplain inundation barriers are reclassified to a relative storm event magnitude based on stage height. High resolution aerial imagery acquired before and after a flood event illuminates the degree of geomorphic modification that occurred in response to a flood event. This analysis allows rates of alluvial landscape evolution to be quantified. Delineating historic river channel avulsion, migration, aggradation, and incision processes will provide critical understanding of the geomorphic response to future flood events. Stream gage data from east and west extents of Yosemite Valley, the documented accounts of flood events by Yosemite National Park, and aerial photographic interpretation serve as checks for the accuracy of modeling and mapping flood inundation in Yosemite Valley.