CHANNEL MORPHOLOGY OF NO THOROUGHFARE CANYON, COLORADO

No Thoroughfare Wash is an un-gauged intermittent stream that drains No Thoroughfare Canyon, the second largest drainage in Colorado National Monument. Despite remaining dry most of the year, late summer and early fall bring intense monsoonal storm systems that often cause flash floods. The goal of this study was to characterize the channel morphology of No Thoroughfare wash in order to better understand the channel-forming discharge regime of the current channel configuration and determine rates of incision into valley fill in the upstream region.

Three study reaches were identified based on changes in channel morphology. The first reach- in the upper canyon- has an average width of 1.15 m and an average slope of 0.022. In the lower canyon, the second reach has an average width of 3.82 m, a slope of 0.025, and a bed material D84 of 45mm. The third reach, downstream from the canyon in alluvium, has an average width of 5.63 m, a slope of 0.018, and a bed material D84 of 56 mm.

The upper portion of No Thoroughfare Canyon has steep walls and is filled by up to 30 meters of unconsolidated stream-terrace alluvium that has been incised by the wash. Cottonwood trees on terraces in the upper canyon were cored in order to derive ages which were used in determining channel incision rates. Results indicate that rapid incision occurred between 1891 and 1939 (0.1 m/yr) but has slowed dramatically since (0.022 m/yr). These results correspond with other studies that show increases in incision rates between approximately 1880 and 1940 across the Colorado Plateau.

Channel forming discharge was estimated in the lower canyon and alluvial reaches by computing the discharge necessary to move the bed material (D84). Critical shear stress was estimated using Shield's equation (τc = τ*c / gD(ρs – ρw)) and the discharge necessary to achieve critical shears stress was modeled using the HEC-RAS software suite. Results show that the discharge necessary to move the cobble sized bed material is 22.5 m3/s for the alluvial reach and 16.5 m3/s in the lower canyon reach, significantly less than the documented 1978 flash flood flow of 263.06 m3/s. This suggests that the current, active, non-vegetated channel is shaped more by high-frequency, low-volume flows than documented peak flash flood flows.