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<br />043,~ <br />be significant, rather than on an exhaustive digest of all available material. A partial <br />list of the docwnents scanned and sampled is attached as Table l. <br />DESCRIPTION <br /> <br />The following description of proposed Narrows Dam and Reservoir is taken from <br />the text in the May 14, 1976, Final Environmental Statement for the project, with minor <br />modification as considered appropriate for this review. <br />Narrows Dam and Reservoir, situated on the South Platte River near Fort Mor- <br />gan, Colorado, as shown on Figure 1, would be the principal feature of the project. <br />Based on a feasibility design, Figure 2, the dam would be a zoned, rolled-earthfill <br />structure containing approximately 8.8 million cubic yards of embankment and 300,000 <br />cubic yards of riprap material for protection of the upstream slope. The downstream <br />face would be covered with 12 inches of topsoil and seeded to grass. The maximwn <br />dam height above streambed would be 147 feet with a crest width of 30 feet and a crest <br />length of 4.2 miles. <br />Under the north abutment north of the South Platte River, a cutoff trench will be <br />excavated to the Pierre shale formation and backfilled with impervious material to con- <br />trol seepage from the reservoir. On the south abutment of the dam, the shale formation <br />slopes down from the river channel. A 20-foot-deep cutoff trench and an 8-foot-wide <br />slurry trench to the shale formation will be constructed to provide seepage control be- <br />low the active conservation water surface elevation of 4399 feet as shown on Figure 2. <br />The south abutment of the proposed Narrows Dam is composed of permeable <br />sand and gravel. Figure 3 shows a geologic cross section of the south abutment ex- <br />tended past Bijou Creek. This section includes data collected in 1976 for use in de- <br />sign of the dam. Seepage losses were estimated to range from 30 cubic feet per second <br />with storage content in the reservoir of 50,000 acre-feet to 60 cubic feet per second <br />with storage content at 300,000 acre-feet. Preliminary results of ground water model <br />studies conducted by the Geological Survey (1976) indicate the possibility of lower seep- <br /> <br />age losses. <br />As a result of seepage, grotmd water levels downstream of the south abutment <br />will rise to levels such that present ground water flow to Bijou Creek and the South <br />Platte River will increase. Most seepage losses are e.xpected to return to the South <br /> <br />2. <br />