Report of Data Pertinent to Lake San Cristobal

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The report which follows was prepared to assist the Hinsdale County Commissioners and the Colorado Water Conservation Board determine a corrective action to protect the outflow channel from further erosion or replace the existing erosion control structure. The existing rock and timber structure was constructed across the outflow channel of Lake San Cristobal as an erosion barrier after less substantial previous attempts were unsuccessful. Although the actual date of construction is unknown, the structure was never intended to be a dam Lake San Cristobal is a natural lake and exempt from state regulation. The timber portion shows signs of deterioration, especially the abutments which could collapse during high flows. Hinsdale County has undertaken some short -term remedial measures by riprapping the abutments and a portion of the channel bottom upstream of the existing structure. A model of the hydrology for Lake San Cristobal and tributary drai nage area indicate the capacity of the outflow channel was equal to 45 percent of the predicted 100 -year, 24 -hour storm The velocities accompanying this flow would cause erosion and scour along the channel bottom and banks in its current condtion. Furthermore, some of the subsurface soils are identifiably susceptable to erosion and must be protected. It would be reasonable to assume the Slumgullion slide was inactive in the vicinity of the outlet for the lake, but movement further upslope has been documented since the mass movement occurred. A cost effective alternative would be placement of additional large, well graded riprap in the channel and along the banks to protect against scour from high flows. In addition the entrance should be smoothed to reduce turbulence. The channel bottom must be kept smooth and the existing width maintained. Regular maintenance would be required.. A more permanent concrete barrier could be constructed to reduce the maintenance, but at a substantially high intiial cost. 9