Flooding after fire: Impacts of the 2013 Colorado Front Range floods on the High Park Fire burn scar

In June 2012, the High Park Fire west of Fort Collins, CO burned 350 km2 of steep forested terrain, leading to elevated runoff and erosion in watersheds draining the burned area. Under the auspices of a NSF RAPID grant we began monitoring precipitation, hillslope-scale sediment production, stream stage, and channel geomorphic change in Skin Gulch and Hill Gulch, two 15 km2 watersheds within the High Park Fire. Short-duration summer thunderstorms are typically the dominant cause of post-fire runoff and erosion in the central and southern Rocky Mountains, but in September 2013 an extreme, long duration storm dropped more than 200 mm of rain in 48 hours. This storm provided a unique opportunity to compare the hydrologic and geomorphic effects of smaller summer thunderstorms to those of the long duration, high magnitude September event. Mean June-August 2013 precipitation in these watersheds was 125 mm, less than half the total for the September 2013 event, but this summer precipitation led to a mean sediment yield of 8 Mg ha-1, about double the mean sediment yield of the much larger September storm. Hillslope sediment production was highest during summer storms that were shorter duration but had higher 5-15 minute precipitation intensities than the September storm. These localized summer 2013 storms led to flashy pulses of flow in the channel network that caused relatively small amounts of channel aggradation or incision. In contrast, the September 2013 event produced sustained high flows that led to substantial geomorphic change throughout the channel network, with more than 2 m of aggradation at the outlet of Skin Gulch. These results indicate that the high intensity summer thunderstorms were most effective at mobilizing sediment from hillslopes, but the more spatially uniform rainfall during the September event produced much more dramatic downstream channel geomorphic change.