Poster Presentation Freshwater Sciences 2023

Revealing how wildfires can affect river sediments and chemistry during droughts and after precipitation (#688)

Alexandra (Lexi) Yokomizo 1 , John Olson 1
  1. Applied Environmental Science, California State University, Monterey Bay, Seaside, CALIFORNIA, United States

With climate change increasing the frequency of wildfires and droughts in California and around the world, it is vital we learn how these events interact. The three major fires burned in central California during the summer of 2020 followed by an abnormally dry winter.  We measured nutrient concentrations and sediment sizes at three sites on the Carmel River downstream of the fires in the summers of 2021 and 2022 to see how droughts change the effects of wildfires on rivers. We examined impacts of wildfires on sediment size and nutrients concentrations using general linear models, and comparing pre- and post-fire data. At the three sites during the 2021 sampling period, the levels of nitrate were not significantly different after the fire whereas phosphate concentrations tripled. However, when samples were taken in 2022, the phosphate levels seemed to return to pre-fire levels while the nitrate levels increased compared to 2021. Despite there being little rainfall to transport nutrients and sediment, increased nutrients and fine sediment occurred downstream of the fires. The distribution of sediment sizes became more bimodal the second year after the fires compared to the first year. The delay of increase of nitrate may have been from fire retardant being sprayed and not from the wildfires. Although water quality and sediment appear to be returning to pre-fire levels, we will continue to monitor them. We will also investigate the role that fire retardants possibly play in causing the observed increase in phosphorus immediately post-fire.