Channel characteristics and large organic debris in adjacent burned and unburned watersheds a decade after wildfire, Park County, Wyoming

Abstract

Relatively few studies of fire effects on streams have examined the period beyond the first few years following wildfire, particularly in the Middle Rocky Mountains. In 1988 the Clover-Mist Fire burned most of the 66.8-km2 Jones Creek watershed, but less than 8 percent of the adjacent 49.5-km2 watershed of Crow Creek. These two watersheds had very similar geologic and pre-fire hydrologic and vegetative characteristics, and few other human activities. This study compared channel conditions between Jones Creek and Crow Creek. The objectives were to: (1) compare channel characteristics and channel stability; (2) compare large organic debris (LOD) characteristics and loading; and (3) identify processes that might explain the observed significant differences between the study streams. Ten reaches of each stream were studied, primarily during summer 1999. On average, the bankfull channel was 1.2 m wider in the burned stream than the reference stream after accounting for differences in drainage area and number of debris jams. Drainage area was the most important factor for explaining mean riffle width. The direction and magnitude of channel-width differences are consistent with the increased duration of near-bankfull discharge in the burned stream and the likely bank erosion after burning. After accounting for reach gradient, graphic mean size of riffle armor was 0.8Φ units coarser on average in Jones Creek than in Crow Creek. LOD frequency in forced-pool-riffle channels is less in Jones Creek than in Crow Creek. Small accumulations of LOD were more frequent along Crow Creek than Jones Creek. The frequency of debris jams was greater in Jones Creek after accounting for differences in either mean piece length or the frequency of pieces longer than channel width. Differences between streams in LOD frequency are consistent with continued greater mobility of debris in Jones Creek, leading to its rapid delivery to downstream reaches and export out of the watershed. Pieces of LOD were smaller and less well anchored in the burned stream than in the reference stream. The combination of larger average piece size and narrower channels yielded a greater ratio of average piece length to channel width in the reference stream. LOD-associated fine-sediment deposits were thicker but less frequent along the burned stream than the reference stream.