The role of large woody debris in modulating the dispersal of a post-fire sediment pulse

•In 2001, a series of post fire debris flows were deposited in a narrow valley.•The affected reaches were surveyed in 2005 and 2012 and then compared.•In 7 years, most of the affected reaches had aggraded due to the increase of LWD.•Reaches with the highest burn classification had the largest increase of LWD.

In 2001, a series of post-fire debris flows brought ~ 30,000 m3 of sediment, deposited as fans, to the narrow valley floor of Sleeping Child Creek in western Montana (USA). In 2005, pebble-counts and surveys of the channel in proximity to six of the debris flow fans documented a regular sequence of fine-grained aggradation upstream of the fans, incision through the fans, and coarse-grained aggradation downstream of the fans. These measurements were repeated in 2012. We found that the delivery of large woody debris (LWD) over the intervening 7 years has been a dominant factor in the disposition of the debris-flow material. The amount of LWD in the study reach has increased by as much as 50% in the areas with a high burn severity, leading to the formation of large logjams that interrupt the flow of sediment along the streambed. Nearly all of the surveyed reaches have aggraded since 2005, including those that had initially begun incising through the debris flow deposits, and the streambed has become generally finer. We hypothesize that, over the next few decades, debris flow sediment not colonized and anchored by riparian vegetation will trickle out of the affected reaches as the logjams slowly degrade.