Analysis of watershed parameters controlling turbidity following the West Fork Complex fire

Abstract

In this thesis, I explore watershed characteristics associated with increased turbidity following wildfires, with the goal of developing relations between turbidity and slope aspect, soil type, slope, and vegetation health from the 2013 West Fork Complex Fire (WFC) in southwest Colorado, USA. Turbidity, precipitation, and stream discharge were previously measured from May to September in 2015 and 2016 in seven watersheds, four burned and three unburned. I then characterized slope, slope aspect, soil type, vegetation, precipitation, and burn severity for each of the seven watersheds, as well as inside the burned areas of the four burned watersheds. I used turbidity as a proxy for sedimentation in each watershed as the dependent variable.Results indicate that from July to September of both 2015 and 2016, burned watersheds had larger spikes in turbidity following precipitation events than unburned watersheds. Higher burn severity and poor vegetation recovery were associated with the strongest positive correlations between total storm volume and turbidity responses. Enhanced Vegetation Index (EVI) was not consistently able to predict which watersheds would experience elevated turbidity following precipitation events; similarly, watershed slope and aspect alone were not able to predict which watersheds would experience larger turbidity responses to precipitation events. Despite seeing significant differences from July to September, during runoff from May to June, there were no significant differences in turbidity between burned and unburned watersheds. These results indicate that drivers of sedimentation in these burned watersheds, for example erosive soils, were more susceptible to precipitation than snowmelt due to exposure to rain splash and the higher intensity precipitation associated with convective rainfall. We also note that if summers continue to extend in the western U.S., burned watersheds with steep slopes, poor vegetation recovery, and erosive soils may experience longer periods of increased sedimentation. Results from this work provide insight on characteristics that determine vulnerability to sedimentation after a wildfire and can help guide land managers in developing effective mitigation strategies.