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regimes and effects of hood stages on the Sandco reach, [hc effects of construction of the seasonal road crossing on the environment, and the effects of lake evaporation on depletions in the Animas River. DESCRIPTION OF STUDY AREA Geology and geomorphology The Animas River valley is one of the largest glaciated valleys in the western United States (Gillam, 1998). The Animas River is sourced in the western San Juan Mountains and flows south to the confluence with the San Juan River at Aztec, New Mexico. Late Pleistocene glaciers covered about 20 perceat of [he Animas River basin and about half of the river's length. Deep canyons and carved valleys have resulted from the glaciation. The Animas Valley north of Durango was carved out to about 1,000 fl below (he current land surface elevation, and the glaciated valley has been filled with sediments and gravels weathered from the San Juan Mountains over hundreds of thou- sands of years..The Animas glacier terminated just north of Durango, leaving the tcnninal moraine along which 32nd Street follows, and the glacier carved oul the Animas valley. After the ice melted, the carved out valley filled with water, clay, sand, and gravel. There were periods during the filling of the valley when swamps and wetlands were predominant, indi- cated by thick clay layers in the Animas valley aquifer that impart poor taste and odors to well waters in [he valley (Cap Allen, oral commun., 1994). Climate The climate of the area is typical of the margins of semi-arid environments bordering large mountain chains. Annual precipitation at Durango averages 19.1 inches, and the mean monthly temperatures range from 27oF in winter to 65oF in summer (Western Regional Climate Center, http://www.wrcc.dri.edu/, accessed June 22, 2004). Vegetation of the study area is a combination of ponderosa pine-Gambe! oak forest on slopes surrounding the valley and riparian cottontvood- willow comnwnities near streams and rivers Mixed in with these vegetation types are pinyon juniper wood- land, mountain shrub,sagcbrush stcppc,and grassland scltings. Hydrology Strcamflow discharges in the Animas River vary widely during the year because of snowmelt runoff during spring, when discharges can increase from 300 ft}/sec at winter low flow to 7,000 ft3/sec during runoff (figure 2). The highest streamflows occurred during monsoonal rainfall events during 1911 and 1926 when the Animas River peaked at about 25,000 and 20,000 ft3/sec, respectively. The lowest flows typically occur during the winter months; however, occasionally amid-winter snowmelt can increase discharges up to 300 or 350 ft3/sec, and a peak of 844 ft3/sec occurred during March 1916. The effects of the sustained drought can be seen by the strcamflow data shown in figure 2. snowmelt runoff peak discharges are typically about 4,000 ft3/sec; however, during May 2002, the peak reached a maximum discharge of 777 ft3/sec (U.S. Geological Survey, hitp:/hvaterdata.usgs.gov/eo/mvis/ uv?09361500, accessed June 22, 2004). Flood frequency calculations were done by the U.S. Army Corps of Engineers specifically for the Sandco Reach of river miles 76.495 to 78.541 (Lee Queen, BLQ, Inc., written commun, 2004). Flood (lows for different intervals are shown in table 1. Table 1. Flood flows and frequencies for the Sandco Reach of the Animas River (Lee Queen, BLQ, Inc., written commun., 2004 Discharge Frequency of occurrence - (ft'/sec) _ 7.5-Year Flood 4,000 10-Year Flood 10,500 50-Year Flood 19,100 700-Year Flood 24,600 RESOURCE USES Historical Uses Gravel mining at the Dalton Pit near the Sandco Plant has taken place since 1987. Sand and gravel were extracted Crom the gravel bar along the east bank S ~,~ $o~~thu~cst Hydro-Logic W°- .,._.