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I I' I I I, 'J I I I I I I I I I ~. I I I ~ I EXECUTIVE SUHMARY , . Peterson lake Dam I s an earth embankment with a maxfmum height of 57 feet and a crest length of 175 feet at elevation 95l7. It fmpounds a reservoir, when full, of 1,252 acre-feet of total storage at the normal reservofr pool elevation of 9510. Releases from the reservoir are controlled by an outlet works that discharges Into an unnamed creek downstream of the darn. A concrete service spillway pipe Is embedded In the left groin with a control elevation of 9510. An auxiliary spillway Is located In a saddle at the northeast corner of the reservoir. It consl sts of a 825-feet long embankment dike, with a crest elevation of 9617, and a 350-feet long rock-armored overflow section at elevation 9511, ne~r the center of the dike. The dam fs 1 oca ted on a sma 11 unnamed tributary of the Cache 1 a Poudre RI ver, approximately eight miles upstream of the Spencer Heights Resort. The drainage basin for Peterson lake Reservoir lies north of Rocky Mountain National Park In the Roosevelt National Forest. The basin divide Is located at approximately 10,500 feet near the headwaters of the Cache la Poudre River. The drainage area Is 2.65 square miles, and the basin has an average slope of 550 feet per mile. The distance from the basin divide to the reservoir Is approximately 1.8 miles. The dam was constructed In the early 1920's for Irrigation supply. The dam has been repaired and modified several times over the years, most significantly In 1972 and 1973 when the crest \1aS lowered by six feet, the service and auxiliary spillways were added, and the outlet works were repatrerl. The dam's operatln9 level has been restricted to elevation 9498 since August 15, 1982 by the Colorado state Engineer's Office (SED). This restriction effectively reduces the reservoir storage capacity to 595 acre-feet. The restriction was imposed due to excessive seepage and the high phreatic surface within the dam resulting In potential Instability of the downstream toe. The City of Greeley unilaterally imposed a zero storage restriction In October of 1985, due to zones of high permeability that were I