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Post-fire dynamics of a gravel bed stream : Fishtrap Creek, British Columbia Phillips, Jeffrey Craig

Abstract

In August 2003, the McLure Fire burned through several watersheds north of Kamloops, including Fishtrap Creek. The burn was extensive, killing almost all the trees in the riparian area. Given our current knowledge, it is not possible to accurately predict channel response to wildfire, the rate of change, or the time scale at which response and recovery may be expected to occur. The purpose of this study is to document the timing and magnitude of initial changes in channel morphology, peak flows, sediment supply, and sediment mobility in Fishtrap Creek in the aftermath of the McLure Fire. We take an empirical approach that combines stream channel monitoring with physically based data analysis. The results of this study have contributed to our understanding of the response of gravel bed rivers to severe vegetation disturbance and have produced an estimate of the timescale at which significant channel transformations may begin to occur. During the summer of 2006 (3 years after the fire), we conducted an intensive field study at Fishtrap Creek. Results indicate that Fishtrap Creek is in a state of transition, triggered by the loss of bank strength and subsequent sediment input to the stream channel. Cross-sectional bed elevation surveys conducted annually from 2004 to 2006 have documented changes in channel form. In localized areas, up to 42 cm of aggradation has been observed; aggradation has typically occurred on the channel bars and behind large woody debris. Avulsions and overbank flooding have also occurred as aggradation forced peak flow stage height higher. Analysis of the relationship between stage height and discharge at a variety of locations indicate that the vast majority of aggradation occurred during the first peak in the 2006 hydrograph. Magnetic tracer stone analysis indicates that sediment mobility was high during the 2006 freshet. The burial depth distribution of tracers suggests substantial scour and a thorough mixing of the active layer. The current trend of channel adjustment is expected to continue until the root systems of emerging vegetation begin to stabilize the banks.

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