Observed Geomorphic Channel Response to Wildfire of Moose Creek, A Spring-Dominated Stream

Abstract

The headwaters of Moose Creek, a spring-dominated stream in eastern Idaho, burned during the Yelluwstone fires of 1988. This study was conducted to determine if post-fire changes in channel form and behavior could be detected. The watershed's geology is predominantly rhyolite ash-flow tuffs, riparian areas are in excellent condition, and past land use includes logging and roads. Areas burned at high fire intensities have been difficult to revegetate, with persistent hydrophobic soils, high surface runoff, and high sediment production. Multiple measurement techniques used to detect channel change and assess response included discharge measurements, pebble counts, channel cross-section suroeys and longitudinal profiles. These measurements were made at upper and lower sites within the watershed between 1985 and 1997. Limited bedload sampling also was conducted at the lower site. The greatest increase in fine bed surface materials (sand and finer) occurred at the lower site shortly after the fires (in 1991), with slightly lower but fluctuating amounts of surface fines since then; post-1991 levels of fines were highest in 1997. At the upper site, results were similar but less distinct because of coarser substrate. Cross-sectional and longitudinal channel surveys provided the greatest information on changes in channel morphology since the fires. At the lower site, flows were confined within the channel, and after initial degradation between 1986 and 1991, the site exhibited net aggradation. At the upper site, continued sediment input and increased frequency of overbank flows in recent years resulted in vertical floodplain accretion. In spite of data deficiencies, evidence suggested that fires have increased the amount of fine sediment in the stream system and that Moose Creek transports excess sediment delivered to it and is constructing a new floodplain where flows exceeding bankfull discharge access the floodplain.