Forest fire, bank strength and channel instability: the ‘unusual’ response of Fishtrap Creek, British Columbia

Abstract

In August 2003, the McLure forest fi re burned 62% of the drainage basin of Fishtrap Creek. Streamfl ow has been measured there since the early 1970s, and suspended sediment concentration and channel morphology have been monitored since the fi re. Although the short post-fi re period (four years) limits our ability to draw fi rm conclusions about streamfl ow changes, there has been no obvious increase in peak fl ows since the fi re. However, the total runoff during the freshet period does appear to have increased and the onset of snowmelt appears to occur about two weeks earlier than it did prior to the fi re. Suspended sediment records from Fishtrap Creek and from an unburnt reference stream nearby are similar, suggesting that the burnt areas have remained relatively stable and that the sediment supply to Fishtrap Creek has not been dramatically altered. In contrast, the stream channel morphology has changed, widening by over 100% of the original width in some places and transforming from a laterally stable plane-bed morphology to a laterally active riffl e-pool morphology. The timing and magnitude of the observed morphologic changes are consistent with the predicted decline in bank strength due to root decay, implying that the observed changes are associated with an internal instability associated with changes to the stream boundaries, rather than with the more typically reported externally driven instabilities caused by changes in streamfl ow or sediment supply. This delayed response in the absence of large changes in streamfl ow or sediment supply, while 'unusual' in that it has not been documented in the previous literature, may be a common mode of response, particularly in wat'ersheds with nival fl ow regimes.

Determining the effects of wildfire on sediment sources using 137Cs and unsupported 210Pb: the role of landscape disturbances and driving forces

“…In Fishtrap watershed, there was limited evidence of mass movement events and gully erosion, and it is concluded that erosion of channel banks represents the main process contributing fine (and coarse) sediment to the creek. This finding is corroborated by the work of Eaton et al (2010a) on the channel bank dynamics of Fishtrap Creek, which has documented significant channel bank erosion and channel migration due to root decay and a lack of vegetative root strength 3-5 years following the wildfire.…”

“…260 km 2 . The fire was classified as a Rank 6 fire, which is the most extreme and hazardous category of fire behaviour (Eaton et al 2010a). The Fishtrap Creek watershed (area is 158 km 2 ; 135 km 2 to the Water Survey of Canada (WSC) gauging station at 51°07′24″N, 120°12′34″W, Fig.…”

“…The watersheds have a distinct snowmelt hydrological regime, with melting typically starting in late March and the main flood discharges (pre-wildfire) occurring from mid April to late May Eaton et al 2010a;WSC 2012), with annual discharge peaks of approximately 5 to 8 m 3 s −1 (Leach and Moore 2010;WSC 2012). River discharge data are available for Fishtrap Creek at the WSC gauging station (WSC (2012), station ID 08LB024, see Eaton et al 2010a) of source materials (n027) was collected from Fishtrap and Jamieson watersheds soon after the 2003 wildfire in spring/ summer 2004 (for details see Owens et al 2006).…”

“…River discharge data are available for Fishtrap Creek at the WSC gauging station (WSC (2012), station ID 08LB024, see Eaton et al 2010a) of source materials (n027) was collected from Fishtrap and Jamieson watersheds soon after the 2003 wildfire in spring/ summer 2004 (for details see Owens et al 2006). A second set of source materials (n096) was collected in summer 2007.…”

“…Such studies have documented an increased contribution from surface soil material in the first few years following the wildfire, often up to contributions of 80-100% in the first year following the wildfire Smith et al 2011b), with a subsequent reduction in surface sources and, thereby, relative increase in subsurface/channel bank sources in subsequent years. In the case of the Fishtrap watershed, this lack of distinct changes in sediment sources (and, in particular, the apparent minimal increase in hillslope erosion) is matched by the lack of a major increase in fine sediment fluxes immediately following the wildfire Eaton et al 2010a;unpublished data). Thus, Petticrew et al (2006) estimated that seasonal (April-October) fine-grained sediment fluxes in Fishtrap watershed in 2004 were low (ca.…”