Geomorphic complexity and sensitivity in channels to fire and floods in mountain catchments

Abstract

Fires and floods are important drivers of geomorphic change. While the hydrologic and geomorphic effects of fires have been studied at the hillslope scale, we have much more limited data on post-fire runoff, channel changes, and inferred or measured sediment storage and delivery at larger scales. In this study we intensively documented channel changes over four years in two very similar ∼15 km2 catchments in the northern Colorado Front Range, Skin Gulch and Hill Gulch, that were burned primarily at high and moderate severity in June 2012. Ten and 11 cross sections and longitudinal profiles along the lower channel network were repeatedly surveyed in Skin Gulch and Hill Gulch, respectively. Summer thunderstorms generally caused deposition in the valley bottoms while incision resulted from the intervening baseflows and spring snowmelt, but a very high intensity summer thunderstorm in Skin Gulch just one week after burning caused much more deposition than in Hill Gulch. Fifteen months after burning both watersheds experienced an exceptional, long-duration flood resulting from a multi-day rainstorm with a several hundred year recurrence interval. This removed nearly all of the deposited post-fire sediment along with some of the older valley bottom deposits. The expansion and coarsening of the channel has greatly decreased the geomorphic sensitivity of both watersheds to future floods, and eliminated the more typical and persistent post-fire depositional signature. The channel changes in Skin Gulch after the fire and long-duration flood were much greater than in Hill Gulch, and this can be attributed to the much greater sediment deposition in Skin Gulch shortly after the fire, reduced geomorphic sensitivity in Hill Gulch resulting from a large erosional flood in 1976, and the spatial distribution of burn severity and storm rainfalls leading to lower peak flows in Hill Gulch. These results suggest that fires in the Rocky Mountains can trigger significant and dynamic hillslope and channel changes over sub-decadal timescales, but unusually long or intense rainstorms can cause larger and more persistent changes regardless of whether a catchment has recently burned.