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..., ro....._", I.i J.j ': I I I I ! be signUicant, rather than on an exhaustive digest of all available material. A partial list of the docwnents sC3.D.D.ed and sampled is attached as Table 1. DESCRIPTION ~Q The followl.og description of proposed Narrows Dam and Reservoir is taken from the te.'"rt in the May 14, 1976, Final Environmental Statement for the project, with minor modlfic:ltion as considered approprlate for this review. Narrows Dam and Reservoir, sItuated on the South Platte River near Fort :r-.'Ior- gan, Colorado, as shown on Flgure 1, would be the principal feature of the project. Based on a feasibilLty desIgn, F.igure 2, the dam would be a zoned, rolled-earthfill structure containing approximately 8. S million cubic yards of emb3I1kment and 300,000 cubic yards of riprap material for protection of the upstream slope. The downstrea.m face would be covered with 12 inches of topsoil and seeded to grass. The maximum dam heIght above streambed would be 147 feet with a crest width of 30 feet and a crest length of 4.2 mU.s. Under the north abutment north of the South Platte RIver, a cutoff trench will be excavated to the Pierre shale formation and backfllled wIth Impervious material to con- trol seepage from the reservoir. On the south abutment of the dam. the shale formation slopes do'WIl from the river channel. A 20-foot-deep cutoff trench and an a-foot-wide slurry trench to the shale formation wm be constructed. to provide seepage control be- low the active conservation water surface elevation of 4399 feet as shown on FIgure 2. The south abutment of the proposed Narrows Dam is composed of permeable sand and gr:lve1. Figu!'e 3 shows a geologIc cross section of the south abutment ex- tended past BIjou Creek. This section includes data collecte~ in 1976 for use in de- sIgn of the dam. Seepage losses were estimated to range from 30 cubic feet per second with storage content In the reservoir of 50.000 acre-feet to 60 cubic feet per second with storage content at 300,000 acre-feet. Prelimlnary results of ground water model studIes conducted by the GeologIcal Survey (1976) lndicate the possibility of lower seep- age losses. As a result of seepage, ground water levels downstream of the south abutment will rise to levels such that present ground water flow to Bijou Creek and the South Platte River will lncrease. Most seepage losses are e."Cpected to return to the South 2.