Relations of Main-Stem Reservoir Operations and Specific Conductance in the Lower Arkansas River

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following the implementation of the 1980 John Martin Reservoir operating plan (Arkansas River Compact Administration, 1980). Similarly, data collected in the upper basin at station 07096000 were evaluated for trends that might have occurred after 1975 because this station was used in the study to evaluate trends in the quantity and quality of inflow into Pueblo Reservoir and the lower basin. Table 1. Main -stem stations for which streamflow and specific- conductance trends were analyzed Station U.S. Geological Period of name Survey station trend number analysis Arkansas River at 07096000 1964 -94 Canon City Arkansas River above 07099400 1966 -94 Pueblo Arkansas River near 07109500 1969 -94 Avondale Arkansas River at 07124000 1961 -94 Las Animas Arkansas River below 07130500 1955 -94 John Martin Reservoir Arkansas River at 07133000 1964 -94 Lamar Description of Study Area The study area includes the Arkansas River Basin in southeastern Colorado from the foothills of the Rocky Mountains near Canon City to Lamar, a distance of about 200 mi (fig. 1). The Arkansas River headwaters are located to the northwest of the study area near Leadville. The river flows south and east through mountainous terrain before emerging from the mountains near Canon City, at an altitude of about 5,350 ft. At Pueblo, the river is impounded to form Pueblo Reservoir. Downstream from Pueblo, the river flows eastward across flat terraces and almost level flood plains, an area commonly referred to as the lower Arkansas River Valley. Immediately downstream from Las Animas, the river is impounded by John Martin Reservoir. About 58 mi downstream from John Martin Reservoir, the river flows into Kansas. The Arkansas River drains an area of about 25,400 mil in Colorado, including 4,669 mil upstream from Pueblo Reservoir. The semiarid climate of the study area is charac- terized by low to moderate precipitation, substantial evaporation, low humidity, moderate to intense winds, and a large daily range in temperature. Mean annual precipitation ranges from 12 in. at Pueblo to 15 in. at Lamar. About 75 to 80 percent of the annual precipitation falls as rain during the growing season. Throughout the area, potential evapotranspiration greatly exceeds precipitation. Land use along the Arkansas River in the lower Arkansas River Basin is predominantly agricultural. Major crops are alfalfa, corn, wheat, and sorghum; about 300,000 acres are irrigated. Most of the irrigated acreage is located in the alluvial valley of the Arkansas River, along the major tributaries, and near off- channel reservoirs. Crop types grown in the valley generally vary downstream by their salinity tolerance. Vegetables and other salt- sensitive crops are grown on proportionally more acreage upstream from La Junta where salinity is lower; alfalfa, which is rela- tively salt tolerant, is grown on proportionally more acreage downstream from La Junta where salinity is much higher (Miles, 1977). The Arkansas River is a partially penetrating stream that is incised into the alluvial deposits that form the valley -fill aquifer of the Arkansas River Valley. The valley -fill aquifer, which extends from Pueblo to the downstream end of the study area, is an unconfined system that directly underlies and is in hydraulic connection with the Arkansas River. The aquifer width varies from 1 to 14 mi and averages 3 to 5 mi. The thickness of the alluvium varies from 0 to about 250 ft. The alluvium consists of fairly well -sorted sand and gravel with minor amounts of clay. Depth to water varies from 0 ft in wetlands in the study area to about 40 ft in eastern Colorado, and the saturated thickness varies from less than 10 ft to about 210 ft. Ground -water flow in the alluvial aquifer generally is from west to east (Hurr and Moore, 1972; Taylor and Luckey, 1974; Nelson and others, 1989a, b, c). Snowmelt from the mountainous upper basin is the primary source of streamflow in the Arkansas River. Snowmelt runoff usually begins in late April or early May and peaks in June. In addition to native snowmelt runoff, streamflow in the Arkansas River is supplemented by the transmountain diversion of water from the Colorado River Basin. Transmountain water is diverted into the Arkansas River Basin at locations that are more than 150 mi upstream from Pueblo INTRODUCTION