Geology and ground-water resources of the lower Bighorn Valley, Montana

The Bighorn River has incised a deep, broad valley in Cretaceous strata along its 65-mile lower reach below the mouth of Bighorn Canyon in south-central Montana. It ceased downcutting at six different levels 100-200 feet apart, widening its flood plain and alluviating each level with about 30 feet of sandy gravel. These deposits are the only economic source of ground water in large areas of the valley where the underlying bedrock consists of relatively impermeable shale to great depths. Ground water in the alluvium is hard and in the irrigated lowlands is highly mineralized at those places where drainage is slow and discharge by evapotranspiration is great. Three bedrock sandstone aquifers are present at moderate depths along three separate reaches of the valley. The sandstones yield soft, moderately to highly mineralized water that contains a high percent sodium. Wells in alluvial gravel of the irrigated lowlands can yield 100 gallons per minute at many places because the alluvium is fairly permeable and is readily recharged by infiltration of applied irrigation water, canal seepage, and ground water moving into the lowlands from the alluvium of tributary coulees. Seepage from the Two Leggins Canal in the central area probably is large. Alluvial gravel deposits have been mantled by thick alluvial and colluvial deposits of silty clay or silt that thin riverward. These fine-grained deposits drain slowly and confine ground water in alluvial gravel under artesian pressure at many places in the irrigated lowlands of the central and southern areas. The piezometric surface is close to the land surface at many places in the central area, and capillary rise and evapotranspiration in waterlogged ground has produced agriculturally harmful alkali deposits. Waterlogging of presently irrigated land in the central area will become more widespread if irrigation is extended to higher terraces to the west unless drainage ditches are installed along the base of high-terrace alluvium to intercept increased seepage and spring discharge. Additional provisions also may be required to intercept water moving through the alluvium of coulees. INTRODUCTION LOCATION AND DESCRIPTION OF ABBA The area included in this investigation extends along the valley of the Bighorn River from the mouth of the canyon in T. 6 S. , R. 31 E. , to the confluence with the Yellowstone River in T. 5 N. , R. 34 E. (fig. 1). The valley crosses parts of Big Horn and Yellowstone Counties, l 2 GEOLOGY, GROUND WATER, BIGHORN VALLEY, MONT. Mont. , and much of it is within the Crow Indian Keservation. The area is 65 miles long and ranges in width from 10 miles at the south end to 3 miles at the north end. It includes the flood plain of the Bighorn River, the adjacent stream terraces, and a part of the bordering shale and sandstone hills. The area lends itself to a three-fold division (fig. 1). The southern area is characterized by a broad irrigated lowland east of the river and a steplike sequence of upland terraces west of the river. It extends from the mountain front to Two Leggins Creek. The central area extends from Two Leggins Creek to 15 miles north of Hardin, where the river is along the eastern edge of the valley and a broad irrigated lowland lies west of the river. The northern area extends from the central area 10 miles to the Yellowstone River valley; it is a narrow lowland between high sandstone hills. PURPOSE AND SCOPE OF INVESTIGATION This report is the result of one of a series of investigations in the U.S. Department of the Interior's program for the development of the natural resources of the Missouri River basin. The U.S. Geological Survey was requested by the U.S. Bureau of Reclamation and the U.S. Bureau of Indian Affairs to investigate the geology and groundwater hydrology of the area as they are related to the availability and quality of ground water for domestic and stock use and as they may affect drainage of presently irrigated land and lands proposed for irrigation. It has been proposed to irrigate about 43,500 acres of additional land on high terraces with water stored behind Yellowtail Dam. Geologic and hydrologic investigations were begun in 1957 by Q. F. Paulson. He was assisted in 1958 by L. J. Hamilton, who completed the field investigations and prepared this report. The geology was mapped on aerial photographs and later transferred to a base map. Altitudes of observation wells were obtained by spirit leveling in T. 5 S. and T. 6 S. , and from quadrangle maps elsewhere in the valley. Most wells and numerous springs were inventoried. The depths to water in 60 observation wells were measured periodically for 4 years and have been measured in 20 selected wells to the present (1966). Pumping tests were made at the sites of five wells to determine the hydrologic characteristics of the aquifers. The chemical quality of water from 19 wells was determined, and the conductivity of water from 145 wells was measured. PREVIOUS INVESTIGATIONS A summary of the geology and natural resources of Big Horn County by Thorn, Hall, Wegemann, and Moulton (1935) was based,