Wildfire, morphologic change and bed material transport at Fishtrap Creek, British Columbia

Abstract

In 2003, a wildfire burned nearly two-thirds of the Fishtrap Creek watershed, including the forested floodplain. Annually repeated surveys of the stream channel — combined with bed material tracer studies in 2006 and 2007 — indicate that the channel has become unstable and that the bed material transport dynamics have changed since the fire. Using these data, the morphologic changes have been quantified and event-scale bed material transport rates have been estimated for four post-fire freshets. The immediate response of the channel (during the 2005 and 2006 freshets) was a progressive shift in channel morphology from a relatively featureless plane-bed morphology devoid of significant sediment accumulations to a riffle-pool morphology in which bars cover a significant proportion of the bed. This transformation occurred in association with moderate bed material sediment transport rates and only minor bank erosion within the study reach. Extensive bank erosion occurred during the 2007 and 2008 freshets, producing a dramatic change in channel morphology and increasing the local bed material transport rates by an order of magnitude. Post-fire monitoring of streamflow and suspended sediment concentrations (a surrogate for upstream sediment supply) indicates that post-fire increases are not detectable (presented in Eaton et al., in press), and our analysis demonstrates that major morphologic changes can occur following wildfire even when the common external drivers of stream discharge and sediment supply remain unchanged.