EVALUATION OF RAPID, POST-FIRE, STREAM CHANNEL VARIATION, DURANGO, COLORADO

The Missionary Ridge Fire burned over 70,000 acres during the summer of 2002 creating unstable conditions conducive to catastrophic debris flow events along the intensely burned steep slopes and high gradient stream channels north and east of Durango, Colorado. Prior to full containment of the fire, an automated, real-time rain-gage network was installed in the burn area by the USGS to monitor local precipitation events. The network consisted of 13 stations equipped with automated tipping bucket rain-gages that provided accurate, site-specific precipitation data. Eleven stream drainages from three separate basins in the burn area were selected for detailed stream channel measurement studies; six in the Animas River drainage basin, one in the Florida River drainage basin, and three in the Vallecito drainage basin. Cross-sectional area of the stream channels was measured between August and October, 2002; measurements were taken following significant precipitation events registered by the rain gages. The discharge measurements were directly compared to the estimated total rainfall over the local drainage basin. Stream channel morphology and massive debris flows were observed following storms with rainfall intensities exceeding 0.15” of precipitation per hour. Several stream drainages registered rapid and dramatic morphologic channel change. In Coon Creek, below the measured section, channel fill in excess of 25 vertical feet from a 1.72” storm was observed. Variations in channel morphology, either rapid incision or deposition, depended upon the local geology, intensity and duration of the each storm event, and the longitudinal position within the drainage channel.