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,~ R. J. Irish Consulting Engineering Geologist, Inc. ENGINEERING GEOLOGIC SITE CHARACTERIZATION STUDY TROUT CREEK DAM SITE AND RESERVOIR AREA CHAFFEECOUNTY,COLORADO INTRODUCTION This report documents and presents the results of our engineering geologic study of the site tentativel y planned for the proposed Trout Creek Dam and for the area i n which its reservoi r would be impounded. The dam, to be constructed es a roller compacted concrete structure, would span a shallow gorge section of the Trout Creek Valley near its entry into the eastern side of the Arkansas River Valley about 3 miles southeast of the town of Buena Vista, Chaffee County, Colorado (Fig. 1). That site is i n the SW 1 /4 NW 1 /4 Sec. 26, T. 14 S., R. 78 W. During the course of our work we have researched published and open-file geologic reports and maps to develop background geologic information pertinent to the project area; monitored the drilling of and logged the core recovered from four borings drilled on the floor of the gorge near the planned damsite; and geologicell y mapped the damsite and reservoir area. The logs of the borings are appended. The USGS's 1:24000-scale Buena Vista East Quadrangle topographic map was enlarged end utilized as the base map for our geologic mapping. Asite-specific map at e larger, more useable scale has not been produced yet. CONSTRUCTION PLAN We understand that the proposed dam would be a roller compacted concrete structure that would be about 50 feet high, as measured above the present stream bed et about El. 7885 feet, thus the dam crest elevation would be about 7935 feet. The alluvial fill across the floor of the gorge, however, appears to be as much as 25 feet thick beneath the creek channel et the damsite and as much as 35 feet thick beneath the alluvial terraces that border the creek channel, thus the constructed height of the dam could approximate 75 feet above the floor of the alluvium-buried bedrock channel of the gorge if the dam were founded wholly on the bedrock end not on the alluvium. The crest of the dam would be about 10 feel wide and about 45 feet long, and would serve as the spillway. The left, or northeastern, abutment of the damsite is s near-vertical granitic rock-faced wall about 75 to 80 feet high. The right, or southwestern, abutment (also faced by granitic rock) slopes upward at about 45 degrees for the first 10 to 12 feet above the floor of the alluvial terrace there, then rises near-vertically for another 1 DO feet or so. The granitic rock ridge that forms