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 the spring of 1958 and 1959 was about 90,000 and 82,000 acre-feet, relatively. 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 rejected. During the 1958 and 1959 water years, total surface-water inflow 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. Move 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 the 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 streams. 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 areas where it is available in sufficient quantity, allowing the surface water to be used in areas of shortage where ground water is not available.
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 was about twice as mineralized at Florence, near the outlet of the valley, as it was at Darby, near the inlet, but the difference is not significant in relation to .the usefulness of the water.
Additional publication details
USGS Numbered Series
Geology and water resources of the Bitterroot Valley, southwestern Montana, with a section on chemical quality of water