Use of geomorphic information in extending the flood record of the West Walker River, California

The paleoflood history of the West Walker River was evaluated in order to extend the flood record beyond the 92-year gaging record. Paleoflood information consisted of both paleoflood deposits and surfaces of non-inundation (non-exceedence thresholds). Radiometric and dendrochronological dating methods were used to place paleoflood information into temporal context. Step-backwater analysis (U.S. Army Corps of Engineers' program HEC-RAS) provided both paleodischarges and non-inundation limits. Evidence from multiple study sites verifies that the 1997 flood event is the largest event to have occurred in several centuries. Flood frequency analysis using the U.S. Water Resources Council method (1981) placed the 1997 flood greater than the 500-year flood event. The Bayesian flood frequency model developed by the U.S. Bureau of Reclamation (FLDFRQ3) was used for flood frequency analysis, as this program allows for incorporation of paleoflood data. The 1997 flood was calculated within the range of the 364- and 468-year events.