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<br />The watersheds of Cement Creek and Mineral Creek, which are tributaries <br />of the Animas River,. cover 20 and 50 square miles, respectively, and <br />consist of high mountainous terrains with steep to moderately steep <br />slopes. These two creeks flow through the town and are the main <br />flooding sources in the Silverton area. <br /> <br />Flood plain development has occurred to a limited extent along <br />Cement Creek in the Town of Silverton. <br /> <br />soils of the Animas River watershed belong to the Rock Outcrop- <br />Cryumbrepts-Cryaquepts Association, according to the U.S. Soil <br />Conservation Service. These soils are shallow and well to poorly <br />drained; therefore, they have slow infiltration rates when thoroughly <br />wetted and a slow rate of water transmission. Most of the watershed <br />is covered with trees and bushes which tend to reduce the runoff <br />potential. <br /> <br />The annual precipitation in the Silverton area is approximately 25 <br />inches, 12 inches of which occurs in the period from May through <br />September. The annual mean temperature is 360F, which varies from <br />550F in July to 170F in January. Humidity is generally quite low, <br />allowing rapid evaporation (Reference 1). <br /> <br />2.3 Principal Flood Problems <br /> <br />A flood potential exists from the melting of the heavy snow cover in <br />the spring. However, rainstorms in fall generally cause severe <br />flooding in the Silverton area. The highway bridges spanning Cement <br />and Mineral Creeks are two troublesome spots where stream channel <br />constriction backs up water, causing flooding in the streets. Other <br />flooding problems are associated with damming created by rock slides <br />and/or debris in the stream channel. <br /> <br />The 1884 flood was the first known flood on the Animas River. In <br />the Town of Silverton, the bridge at Reese Street across Cement <br />Creek was washed out and nearby houses were threatened with being <br />swept away. <br /> <br />The maximum flood of record on the Animas River occurred in 1911. <br />On Cement Creek, high waters broke over banks just above the highway <br />bridge and flooded the main street. Bridges at Gladstone Depot and <br />two others on upper Greene Street were destroyed. Railroad tracks <br />and bridges in the general area were washed away, and roads were <br />blocked by rnudsliqes and rockslides in many places, so that the <br />entire Town of Silverton was isolated. <br /> <br />The second highest flood of record occurred in 1927. The railroad <br />along the Animas River was flooded up to Howardsvi1le and railroad <br />tracks and roadbed were washed out, taking four weeks to restore <br />train service. The 1970 flood caused railroad damage of approximately <br />$500,000. Floodwaters ripped up railroad tracks and demolished the <br /> <br />5 <br />