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Silver Jack Dam

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Silver Jack Dam is located on Cimarron Creek about 20 miles above the junction with the Gunnison River. The rolled-earthfill dam contains 1,278,140 cubic yards of material and has a structural height of 173 feet. Its crest is 1,050 feet long and 30 feet wide. The outlet works to Cimarron Creek in the right abutment has a capacity of 280 cubic feet per second with the reservoir at the normal water surface elevation of 8926.0 feet and a capacity of 160 cubic feet per second at the minimum water surface elevation of 8840.0 feet. The spillway on the right abutment is an uncontrolled ogee section with a capacity of 6,220 cubic feet per second at maximum water surface elevation. The reservoir has a total capacity of 13,520 acre-feet, including 12,820 acre-feet of active capacity and 700 acre-feet of inactive capacity. When filled to its normal water surface elevation, the reservoir has a surface area of 293 acres.

Geology

Silver Jack Dam is constructed on glacial till (Qwt) and alluvial stream deposits (Qal).  Approximately 20 feet of pervious alluvial gravel overlies at least 80 feet of relatively impervious glacial till.  The alluvial deposits blanket the valley bottom along Cimmaron Creek from approximate station 6+10 to 10+50.  The pervious alluvial gravel deposits were excavated from the cutoff trench, which is located roughly 150 feet upstream of the dam centerline and approximately 100 feet wide.  Alluvial gravel deposits remain beneath the dam upstream and downstream of the cutoff trench.  The alluvial material is crudely stratified and predominantly clean to silty, sandy gravel and was logged in the wells of the cutoff trench as "sandy gravel, cobbles and boulders".  These alluvial deposits also contain occasional thin layers of organic materials.  Laboratory analysis of a 30-pound sample of alluvium from the cutoff trench indicated .5 percent nonplastic fines, 22 percent sand, 74 percent gravel and 3.5 percent cobbles.  Isolated, small pockets or lenses of sand were noted on the maps of the cutoff trench walls in the alluvium.  The contact between alluvium and the glacial till was also mapped in the cutoff trench.  The underlying glacial till deposit consists predominantly of widely varying amounts of silt, gravel, cobbles and boulders and was described in the cutoff trench as "clayey to silty gravel and cobbles with blocks of shale and agglomerate".  Laboratory analyses of glacial till material from the cutoff trench indicate 20 percent fines (liquid limit 36, plasticity index 10), 43 percent sand and 37 percent gravel.  The large blocks of rock within the foundation glacial deposits have been attributed to old landslide activity.


Last updated: Jul 22, 2009