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<br />The Uncompahgre River headwaters rise on the northern slopes of <br />the San Juan Mountains, west of the Continental Divide, at eleva- <br />tions higher than 13,000 feet. The river then flows northwesterly <br />through Ouray, Montrose, and Delta Counties for approximately 70 <br />miles to its confluence with the Gunnison River near the City of <br />Delta. The Uncompahgre River drains approximately 1,100 square <br />miles at its confluence with the Gunnison River. <br /> <br />Cascade and Portland Creeks drain tributary watersheds of 1.3 and <br />2.6 square miles, respectively. The watersheds lie within a circular <br />basin east of Ouray that is appropriately named "The Amphitheater. <br />The area is encircled by jagged peaks that reach elevations of <br />12,700 feet and that have essentially bare rock faces. More than <br />one-half of the watersheds contain no vegetation, which combined <br />with the steep slopes results in extremely high rates of runoff <br />when storm clouds are trapped in the watershed area. In the process <br />of falling 4,000 feet in elevation to the City of Ouray, the flow <br />in the creeks reaches violent velocities that carry sand, gravel, <br />rocks, trees, and large boulders into the city. This process of <br />violent fragmentation and erosion of the Amphitheater has, over a <br />period of centuries, deposited an "alluvial fan" of boulders in <br />the Uncompahgre Valley that resembles a large rock dome at the <br />mouths of the two creeks. The City of Ouray is constructed upon <br />this alluvial fan and is exposed to flows that, when they reach <br />the fan, attempt to spread out, flatten, and follow circuitous <br />routes to the Uncompahgre River, washing through the city along <br />frequently unpredictable paths. <br /> <br />The climate within the Uncompahgre River basin is semiarid, but <br />rainfall and temperatures vary widely. Average annual precipitation <br />ranges from 6 inches at Delta and 13 inches in the Colona-Ridgeway <br />area to as much as 40 inches in the mountainous areas. Approximately <br />30 to 40 percent of the precipitation is snowfall. The area is <br />subject to cloudburst activity. The frost-free period averages <br />approximately 127 days annually, and varies from 112 days at higher <br />elevations to 148 days in the valley. <br /> <br />Vegetation in the area consists of pinon, juniper, sagebrush, oak <br />brush, and ponderosa pine. <br /> <br />The topography surrounding Ouray ranges from moderately rolling to <br />high semimountainous terrain, with a comparatively narrow river <br />valley within OUray. The soils are coarse grained, mostly of <br />alluvial origin. <br /> <br />Development within the detailed-study area has been primarily mobile <br />home parks. <br /> <br />2.3 Principal Flood problems <br /> <br />Floodflows on the Uncompahgre River within the study area result <br />from the rapid melting of the mountain snowpack during May, June, <br />and early July. Snowmelt is occasionally augmented by rainfall. <br /> <br />4 <br />