THE RELATIONSHIP BETWEEN LARGE WOODY DEBRIS AND A DECREASE OF MEDIAN BED MATERIAL SIZE IN A GRAVEL BED CHANNEL AFTER POST-FIRE DEBRIS FLOWS

To further the understanding of how rivers process an instantaneous increase in sediment supply, a study was conducted of a gravel bed channel affected by post fire debris flows. Sleeping Child Creek, a tributary of the Bitterroot River, is located in the Sapphire Mountains of west central Montana. In the summer of 2000, a large fire burned 1500 acres of the watershed. The following summer, thunderstorms brought intense rainfall to the area, triggering a series of debris flows originating from the main tributaries that had burned. Along the 10-kilometer study reach, cross sections and pebble counts were taken near six debris flow fans. A channel survey of large woody debris was also conducted. The initial hypothesis was that the fine sediment would disperse downstream, exposing coarse bed material. Comparing the cross sections with previous measurements has shown that the river has aggraded throughout most of the site. The bed material has become finer and there is about ten times more large woody debris than conducted six years ago. The aggradation and the finer bed material can be attributed to the increase in large woody debris in the channel. The response documented in Sleeping Child Creek informs our understanding of channel evolution after large sediment pulses. In burned landscapes, the role of large woody debris could be instrumental in modulating the storage and release of the post-fire sediment pulse.