HISTORICAL SEDIMENT BUDGET FOR THE BLACK CANYON OF THE GUNNISON NATIONAL PARK, COLORADO

The Black Canyon of the Gunnison is a narrow and steep canyon located near Montrose, Colorado along the Gunnison River. The fluvial network is a mixed gravel and bedrock main channel with ephemeral side tributary hollows. Flow rates are completely controlled immediately upstream by a diversion tunnel and three reservoirs. The management of the hydraulic control structures has decreased low frequency, high stage flows. Such flows are the dominant geomorphic force in bedrock channel systems. The object of this study is a sediment budget analysis of the Black Canyon of the Gunnison and possible changes over time due to flow regulation. A historical comparison of the area extent and volume of river bank storage of alluvium and colluvium sediment sources such as talus slopes and debris flows is done using historical area photography in a geographic information system. This analysis shows that the surface area and volume of colluvium sediment sources changed between 1939 and 1992. A simple sediment transport model of colluvium along the main channel reach is developed using the ratio between bed shear stress averaged over an individual reach and critical shear stress. Sensitivity analyses for Manning’s n and the coefficient for critical shear stress in model are conducted. The likelihood of any given reach to be transporting incorporates a range of shear stress ratios corresponding to different paired values of Manning’s n and the coefficient for critical shear stress. The sensitivity of Manning’s n in the model is lower than the sensitivity of the coefficient used in calculated critical shear stress. Comparison of model results to previous studies of sediment transport for a given flow rate shows agreement. Model results suggest that a majority of reaches are transporting colluvium from sediment sources at flows above a recurrence interval of 2.5 to 4 years post-regulation as opposed to 1.09 to 1.17 years pre-regulation. However, upstream and downstream regions with lower bed slope transport a majority of the time at a recurrence interval of 6 to 10 years post-regulation as opposed to 1.3 to 1.5 years pre-regulation. The frequency of flows initiating motion of river bank sediment along the main channel has declined since regulation.