Blown away: 3-year flood period yields 20 times the sediment export rate as previous flood and drought periods in a regulated gravel/cobble river

Recent remote sensing advances enable interannual to decadal monitoring of fluvial topographic change at one meter resolution. By differencing four 1-m digital elevation models (DEMs), three topographic change detection (TCD) analyses were created for the 37.1-km lower Yuba River (LYR) in northern California over 18 years (1999 to 2017). The study addresses whether the channel incised or aggraded, and whether patterns of scour and fill magnitudes remained consistent in flood versus drought periods. Epochs 1 (1999-2008) and 3 (2014-2017) were wet with large floods of long duration over bankfull. Epoch 2 (2008-2014) was a drought period with only a few short, moderate floods. Three TCDs were performed using a spatially explicit uncertainty analysis method yielding a 95% confidence level threshold per pixel. Area, volume, and change depth were evaluated at three spatial scales: entire LYR, geomorphic reach, and morphological unit. In-channel versus overbank analysis revealed the channel was net depositional for all survey epochs, while the overbank area was net erosional. These results indicate that the LYR is not incising but is in fact well connected to its floodplains, with the channel scouring and depositing laterally as it migrates. While some patterns were consistent across the epochs, the net sediment export rate did not scale with discharge as one might expect. In epoch 1, only 1.70 x 10⁴ m³/year of sediment was exported out of the LYR, but this volume increased to 2.22 x 10⁴ m³/year during the drought period of epoch 2, suggesting that lower flows can cause more erosion in the LYR. However, in the second flood period of epoch 3, the LYR experienced a net erosional export rate of 4.56 x 10⁵ m³/year, more than 20 times greater than the export rate seen in the first flood epoch, even though epoch 3 had a lower peak discharge. These four high resolution DEM datasets capturing nearly two decades of change permitted a rare opportunity to describe long term geomorphic change in a regulated gravel/cobble river. The findings from the last survey epoch in this study added support as well as contradiction to some of the initial theories regarding the observed erosion and deposition in the LYR, emphasizing the importance of long term monitoring to better explain and predict topographic change drivers.