Effects of vegetation disturbance by fire on channel initiation thresholds

The disturbance or removal of vegetation by wildfire influences channel incision following intense rainfall events. Here we empirically examine relationships between the severity of vegetation disturbance and geomorphic controls on threshold conditions that lead to channel incision. We conducted post-fire field mapping and digital spatial analyses across 97 recently formed channel heads in the Rocky Mountains of Montana and Idaho, USA, to identify the relationship between remotely-sensed fire severity and vegetation disturbance and the source area and gradient conditions required for channel initiation. We found that the relationship between the size of source areas and source-area steepness was described by an inverse power function, consistent with established theory, across the range of fire severity, but that the magnitude of the slope–area relationship was significantly correlated with increasing fire severity. Further, at higher levels of fire severity, source areas above channel heads had lower slopes and somewhat larger areas. The findings suggest that the onset of channel incision defined by location of channel heads is controlled by fire severity and that the threshold for channel initiation decreases as vegetation disturbance increases. We also found that, in a subset of catchments for which LiDAR data were available, total curvature explained channel head location across the range of fire severity, with a small but significant contribution from source area steepness. Steepness remains more important at lower fire severity, however, and total curvature dominates where fire severity is most extreme. This suggests that forces of convergent flow are not fully expressed until a significant proportion of vegetation has been consumed such that flow resistance is minimized. Our findings, and the use of a continuous fire severity metric, contribute an ecohydrological and biogeomorphical template for studies of post-fire geomorphic responses and landscape evolution.