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5M <br />I <br />Lake San Cristobal was created as a result of the massive Slumgullion slide <br />which blocked the Lake Fork of the Gunnison River above Lake City. Early <br />attempts to control the lake level and curb erosion in the outflow channel <br />were not too successful and the failure of a log and rock control weir <br />purportedly caused damage to downstream properties. <br />Working with Mr. Fred Paddock, Division 4, Water Engineer at that time, the <br />Hinsdale County Board of Commissioners constructed a more substantial timber <br />crib and rock fill structure. The timber and rock erosion control structure <br />was never intended to be a dam. The structure is similar to plans depicted on <br />a drawing found in the Division 4 office prepared by Mr. Paddock. The actual <br />date of construction is not known. <br />The concern for the adequacy of the existing control structure was brought to <br />the attention of the Hinsdale County Commissioners when new piers were <br />constructed for the county road bridge, located about 100 feet downstream. <br />The bridge engineer indicated the condition of the rock /timber abutments at <br />the control weir were not adequate to withstand large flows. Failure of the <br />abutments could jeopardize the integrity of the bridge. <br />Based on the bridge engineer's opinion, emergency remedial action was <br />undertaken early in the spring of 1990. The work consisted of placing large <br />boulders (2 to 3 feet diameter) across the river channel immediately upstream <br />of the existing timber control structure. Mr. Robert Hurd, Hinsdale County <br />Road Department Supervisor, indicated the normal lake level was approximately <br />10 to 12 inches lower after the 1984 runoff. Apparently, the top timbers on <br />the weir broke loose and the rock washed away during the 1984 spring runoff. <br />The placement of the large boulders in 1990 not only raised the lake level <br />another 2 feet, but the runoff peaked about 2 feet above the top of the <br />boulders. The high water apparently created some difficulties around the lake <br />with boat docks, foot bridges, and inundated fish habitat. 2 After the runoff, <br />it was observed the water flow over the boulders scoured a hole approximately <br />18 inches deep and extended along the left upstream side of the timber <br />structure about 6 feet. <br />