POST-FIRE EROSION AND SEDIMENT DELIVERY RATES TO HEADWATER STREAMS IN THE CALIFORNIA COAST RANGES

The 2015 Valley Fire in Lake County California burned 76,067 acres in September 2015, and was declared a Presidential disaster area. The burned area included 99% of the 3,493 acre Boggs Mountain State Demonstration Forest, which is underlain mostly by relatively young volcanic rock composed of andesitic flows and lahar deposits associated with the Quaternary age Clear Lake Volcanic field, and to a lesser extent by Mesozoic sedimentary rocks of the Great Valley sequence. We instrumented six headwater catchments with varying degrees of burn severity prior to the 2015-2016 winter wet season to measure runoff and erosion. We also measured organic and inorganic (e.g., rock) ground surface cover and mapped rill networks within each small (~0.5-1 ac) catchment. The amount of surface cover decreased with increasing burn severity and ranged from 62% in the least severely burned catchment to 32% in the most severely burned catchment.

Results from initial runoff events show that runoff, erosion, and sediment delivery increase with increasing burn severity, and the more severely burned catchments produced the most surface runoff, highest peak flow rates, and largest sediment yields. The number of rills per unit area also increased with increasing burn severity, indicating greater runoff and sediment delivery from the hillslopes to the channels. Our results provide magnitudes of burned watershed responses to varying rainfall conditions in the California Coast Ranges. Relationships between surface cover, rill density, runoff, as well as sediment production and delivery rates are important for guiding post-fire management and determining watershed impacts, and provide additional insights to erosion rates in the California Coast Ranges. Ongoing work includes continued monitoring of these processes. Future work will include comparing sediment production and delivery from different post-fire logging and reforestation activities.