A study of the geology and the groundwater conditions of the Helena Valley, in the west-central part of Montana, was made during the summer of 19*^80 The purpose of the study was to gather data on the occurrence of ground water and its relation to present and proposed irrigation in the area. With such data one may better predict the effects of the application of irrigation water to additional lands in the area. 2 GEOLOGY AND GROUNDWATER RESOURCES OF THE HELENA VALLEY, MONTANA Ground vaters in the valley contain moderate amounts of dissolved solids, the predominant ions being calcium, bicarbonate, and sulfate, Little difference is discernible in the composition of waters in the Quaternary and Tertiary deposits; the dilute nature of these waters indicates that the valley fill is derived from formations containing only small amounts of soluble minerals. The quality of the ground water appears to be little affected by recharge of the groundwater reservoir from return irrigation flows. Waterlogged areas may in time show an increase in mineralization as a result of evaporation. Both ground and surface waters are classified as "excellent to good" on the basis of Wilcox's criteria for irrigation waters. Boron, percent sodium, and dissolved solids are relatively low. Waters used for drinking or domestic purposes are generally considered satisfactory except, in some instances, for hardness. INTRODUCTION Purpose and Scope of Investigation The purpose of this investigation was to gather data on the occurrence, conditions, and potentialities of the ground water in the Helena Valley. Special consideration was given to the source of the ground water, to its direction of movement, to the fluctuations of water levels in wells, to the depth to the water table below the land surface, to the available supply, and to the present and potential extent of waterlogging and means for alleviating it. This study was a part of the program of the Interior Department for development of the Missouri River basin, and it is directly related to the proposed Canyon Ferry project which calls for diversion of water from the Missouri River into the Helena Valley to irrigate about 10,000 acres of land not now irrigated and to furnish additional water to about 5*000 acres of land already irrigated. This report is based principally on field work done by Howard W. Lorenz between July and early November 19^8. However, water-level measurements made during April, May, and June, 19^8, by the Bureau of Reclamation are incorporated in this report. The geology of the area was mapped on aerial photographs, and the field data were later transferred to a base map by use of a sketchmaster. An inventory was made of all wells in the area; water levels in observation wells were measured periodically; and a watertable contour map was constructed from instrumentally determined altitudes of water levels in wells. The field investigation was under the general supervision of A. N. Sayre, chief of the Ground Water Branch of the Federal Geological Survey, and G. H. Taylor, re...

The Townsend Valley in west-central Montana is a 600 square-mile intermontane basin bordered by the Big Belt Mountains on the east and by the Elkhorn Mountains on the west. It extends from approximately T. 4 N., to T. 11 N., inclusive. Rocks ranging in age from Precambrian to Cretaceous crop out in the mountainous areas that surround the valley, and rocks of Tertiary (Oligocene and Miocene) age crop out on the higher valley slopes and underlie the alluvial deposits in the valley bottom. The area is drained by the Missouri River, which enters the Townsend Valley through a gorge at the southern end of the valley and leaves through another gorge at the northern end. The climate of the valley is semi arid; the average annual precipitation is 12.35 inches. Farming is the principal occupation. About one-third of the cultivated land is irrigated by water diverted from the Missouri River and perennial mountain streams, and the remaining cultivated land is farmed by dryland methods. Wheat, oats, barley, sugar beets, and potatoes! are the principal crops. Mining, which was once the chief industry of the area, is now (1951) sporadic and on a small scale. Gold, silver, and lead are the chief metals mined. Small amounts of coal, marble, quartzite, and iron ore have been mined during the past 10 years. A large reservoir of ground water underlies the valley. On the valley floor shallow well s yield an abundant supply of water for domestic and stock use, but on the higher slopes wells must be drilled to a depth of 200 to 300 feet to obtain an adequate supply. Water is confined under artesian pressure in Tertiary beds that underlie the southern end of the valley and the area along the west flank of the Dry Creek anticline east of Townsend. The denser pre-Tertiary formations contain water in fissures and small openings and springs are numerous where these rocks crop out. Big and Plunket (Mockel) Springs, which yield about 57 and 8.7 cfs, respectively, are the largest. The principal sources of recharge to ground water in the younger, unconsolidated deposits are streams, irrigation canals and laterals, and irrigation floodwater. About 8,500 acres of the land is waterlogged because the transmissibility of the underlying waterbearing materials is low or because the configuration of the bedrock is such that the movement of ground water is restricted. Of this total, 1,500 acres will be inundated by water impounded by the Canyon Ferry dam. Drainage of the remaining waterlogged land would greatly increase the agricultural potential of the valley. Irrigation utilizing groundwater resources is in a pioneering stage in the Townsend Valley. In the summer of 1950 the first well for irrigation supply was drilled 3% miles southeast of Townsend and when tested, using air compressors, produced about 240 gpm. 171 172 CONTRIBUTIONS TO HYDROLOGY Water from ground and surface sources in the Townsend Valley generally is hard bat only moderately mineralized and is suitable for most domestic and irrigation uses. The range in dissolved solids and hardness...