UC Riverside

Post-wildfire hydrological regimes can result in a dramatic increase in watershed sediment yield, particularly fine sediments (diameter < 62.5 µm) from small mountainous watersheds. The objective of this study was to evaluate changes to fine suspended sediment concentration-discharge relationships in the lower Ventura River, CA after 82% of the watershed burned in the 2017 Thomas Fire relative to persistent periods of time-varying suspended sediment dynamics over the previous four decades. We monitored fine suspended sediment concentrations during stormflow events in the Ventura River during water years 2018 and 2019, and characterized post-wildfire suspended sediment concentration-discharge relationships, which were then compared with pre-wildfire conditions monitored by the USGS from 1960 to 2015. Results show that 2018 and 2019 storm flow suspended sediment concentrations were 13.2 and 4.6 times higher, respectively, than predicted on the basis of the entire USGS monitoring record, but 32.5 and 9.5 times higher than the most recent period of persistently low suspended sediment concentrations. In comparison, previous decadal-scale periods of persistently high and low suspended sediment concentrations were 2.3 and -2.2 times that of the total USGS historical record. However, overall flux during post-wildfire years were far below the long term average because of relatively low precipitation in 2018 and 2019. These findings highlight the importance of post-wildfire storm event magnitudes for long-term sediment yield in small mountainous watersheds.