UC Davis

Floods play a critical role in geomorphic change, but whether peak magnitude, duration, volume, or frequency determines the resulting magnitude of erosion and deposition is a question often proposed in geomorphic effectiveness studies. This study investigated that question using digital elevation model differencing to compare and contrast three hydrologically distinct epochs of topographic change spanning 18 years in the 37-km gravel-cobble lower Yuba River (LYR) in northern California. Scour and fill were analyzed by volume at segment and geomorphic reach scales. Each epoch’s hydrology was characterized using 15-minute and daily averaged flow to obtain distinct peak and recurrence, duration, and volume metrics. Epochs 1 (1999-2008) and 3 (2014-2017) were wet with large floods reaching 3,206 and 2,466 m3/s, respectively, though of different flood durations. Epoch 2 (2008-2014) was a drought period with only four brief moderate floods (peak of 1,245 m3/s). Total volumetric changes showed that major geomorphic response occurred primarily during large flood events; however, total scour and net export of sediment varied greatly, with 20 times more export in epoch 3 compared to epoch 1. The key finding was that greater peak discharge was not correlated with greater net and total erosion; differences were better explained by duration and volume above floodway-filling stage. This finding highlights the importance of considering flood duration and volume, along with peak, to assess flood magnitude in the context of flood management, frequency analysis, and resulting geomorphic changes.