The Role of Large Woody Debris in Inhibiting the Dispersion of a Post-Fire Sediment Pulse

Abstract

THE ROLE OF LARGE WOODY DEBRIS IN INHIBITING THE DISPERSION OF A POST-FIRE SEDIMENT PULSE by Lauren E. Short To further the understanding of how rivers process an instantaneous increase in sediment supply, a study was conducted on a gravel bed channel affected by post-fire debris flows. Sleeping Child Creek (SCC), a tributary to the Bitterroot River, is located in the Sapphire Mountains of west central Montana. In the summer of 2000, 1500 acres of Sleeping Child Creek’s watershed were burned with 65% of the area classified as a high severity burn. The following summer, intense rainfall triggered debris flows originating from tributaries of SCC with headwaters toward the north. Along a 7-km study reach, cross sections, longitudinal profiles, and pebble counts were taken near six debris flow fans in the summer of 2012. In addition, a survey of large woody debris was conducted throughout the study reach. Comparing the cross sections with previous measurements taken in 2005 demonstrated that the river has aggraded throughout most of the site. Moreover, the bed material has become finer and the amount of large woody debris has doubled in seven years. The aggradation and the finer bed material can be attributed to the increase in large woody debris in the channel. In 2005, the large woody debris in the channel was from the debris flows. Since then, the large woody debris has come from the severely burned slopes adjacent to the channel. The response documented in Sleeping Child Creek demonstrates that, in burned mountainous landscapes, the large woody debris can be instrumental in modulating the storage and release of the post-fire