Wildfire Impacts on the Persistent Suspended Sediment Dynamics of the Ventura River, California

Focus: After wildfire, small mountainous watersheds can produce dramatic increases in fluvial sediment transport, but these changes are often evaluated under the assumption of stationary prefire sediment dynamics. The objectives of this study were to investigate temporal dependence in the fine (silt and clay) suspended sediment dynamics of the Ventura River before and after 48.5 % of the 194 km2 undammed portion of the watershed burned in the 2017 Thomas Fire. New hydrological insights: Fine sediment concentrations in the Ventura River decreased between 1971 and 2008. In 2018 and 2019, fine suspended sediment concentrations were 14.2 and 5.1 times higher than predicted based on the entire monitoring record, but 32.4 and 9.5 times higher than the most recent period of persistently low concentrations. The Thomas Fire resulted in a 200x increase in fine sediment flux in 2018 relative to pre-fire conditions. However, fine suspended sediment flux during both post-fire years, 2018: 0.073 (0.050-0.105) MT and 2019: 0.393 (0.284-0.447) MT, was comparable to the long term average 0.177 (0.140-0.230) MT because of relatively little post-fire precipitation. These findings highlight the importance of considering preexisting time-dependent behavior when characterizing the fire impacts on suspended sediment dynamics, and the critical role that storm event magnitudes play in determining the sediment yield of small mountainous watersheds after wildfire.