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The Bitterroot Valley is a Late Cretaceous structural basin that was partly filled at its deepest point by more than 1,640 feet of Tertiary sediments. These sediments grade valleyward from coarse colluvial deposits along the edges of the valley to fine-grained deposits and then to coarse channel deposits of the ancestral Bitterroot River near the center of the valley. Beneath the flood plain and low terraces of the present Bitterroot River, about 40 feet of Quaternary alluvium overlies the Tertiary sediments. Each spring and summer, at rates greatly exceeding discharge, water infiltrates to the ground-water reservoir in the Tertiary and Quaternary rocks. During the fall and winter, water is released from storage. Net recharge in tine spring of 1958 and 1959 was about 90,000 and 82,000 acre-feet, respectively. Net discharge during the rest of each year was about 90,000 and 76,000 acre-feet, respectively. Some surface water available for recharge during high runoff each spring is rejected. During the 1958 and 1959 water years* total surface-water inflow was about 1.7 million and 2.0 million acre-feet, respectively. Consumptive use during these water years was about 450,000 and 400,000 acre-feet, respectively. More pumping from the ground-water reservoir would provide additional storage space for peak runoff and would increase the potential consumptive use in the valley. Additional wells, capable of yielding more than 250 gpm (gallons per minute), can be constructed on the flood plain of tine Bitterroot River and on some of the adjacent low terraces, especially those east of the river. Near Corvallis, on a low terrace, wells capable of yielding 1,000 gpm or more can be constructed. Wells capable of yielding 50 to 250 gpm can be constructed on many of the alluvial fans of the tributary stream®. In the remaining area, wells will generally yield only enough water for domestic and stock use. From the hydrologic standpoint, the best use of ground water for irrigation is conjunctive use with surface water. Surface water is adequate early in the season and can be distributed throughout the area. As shortages occur, ground water can be used in area® where it is available in sufficient quantity, allowing the surface water to be used in areas of shortage where ground water is not available. 2 GEOLOGY, WATER RESOURCES, BITTERROOT VALLEY, MONT. 'Water in the Bitterroot Valley is of satisfactory chemical quality for domestic, stock, municipal, and most industrial uses. Surface water is softer, as a rule, and contains less dissolved solids than the ground water. Streams heading in the Sapphire Mountains are more mineralized than those heading in the Bitterroot Mountains. Bitterroot River water in October 1955 wasi about twice as mineralized at Florence, near the outlet of the valley, as it wasi at Darby,, near the inlet, but the difference is not significant in relation to the usefulness of the water. INTRODUCTION A cooperative program, for the evaluation of the ground-water resources in Montana was begun in July 1955 by the U.S. Geological Survey and the Montana Bureau of Mines and Geology. The appraisal of the water resources of the Bitterroot Valley was one of the first projects started under the cooperative agreement. The main objectives were to determine (1) character and extent of the water-bearing materials; (2) occurrence, direction of movement, and availability of ground water; (3) annual, seasonal, and long-term fluctuations of the water table; (4) surface-water and ground-water inflow to and outflow from the valley in space and time; (5) areas from which substantial supplies of ground water of good quality can be obtained; and (6) chemical quality of the water. LOCATION AND EXTENT The Bitterroot Valley extends from near Darby to Florence and is bounded on the east by the Sapphire Mountains and on the west by the Bitterroot Mountains. The valley is about 45 miles long, averages about 7 miles wide, and includes about 300 square miles (fig. 1). In 1955 and 1956, geologic and hydrologic data were collected east of the river. In 1957, the study area was expanded to include the western part of the valley, and the scope of the investigation was expanded to include surface-water inflow and outflow. WELL-NUMBERING SYSTEM The wells described in this report are assigned numbers on the basis of their location within the U.S. Bureau of Land Management's system of land subdivision. The well number shows the location of the well by township, range, section, and position within the section (fig. 2). The first letter of the well number gives the quadrant of the meridian and base-line system in which the well is located. The first numeral of the well number denotes the township, the second the range, and the third the section in which the well is located. Lowercase letters following the section number show the location of the well within the quarter section and the quarter-quarter section, respectively. Lowercase letters are assigned to the quarter or quarter-quarter

Geology and water resources of the Bitterroot Valley, southwestern Montana, with a section on chemical quality of water