The Missouri Basin Region lies in the north-central part of the United States and southern Canada. It includes parts of Alberta and Saskatchewan in Canada; parts of Montana, Wyoming, North Dakota, South Dakota, Minnesota, Iowa, Colorado, Kansas, and Missouri, and all of Nebraska in the United States. The region includes about one-sixth of the contiguous United States and requires large water supplies for irrigation, industrial, public, and rural uses. Climate ranges from semiarid to subhumid. Normal annual precipitation increases generally eastward in the downstream direction, but precipitation is not a dependable source of supply. The Missouri River and its tributaries furnish water to many users, but surface water is often inadequate to meet large demands. Numerous surface reservoirs help to regulate streamflow and provide storage, but they also allow an increase in evapotranspiration, which in some areas exceeds normal precipitation. Ground water occurs in aquifers classified as alluvial deposits of sand and gravel, glacial deposits, dune-sand deposits, basin-fill deposits of sand and gravel, sandstone, siltstone, fractured sandy clay, limestone, and dolomite. Ground water can be developed and managed in an orderly manner provided adequate geologic and hydrologic data are available to determine aquifer characteristics and response to pumping and other hydraulic stresses. These data and determinations are essential to design, testing, and implementation of water management plans.

Unconsolidated and semiconsolidated aquifers include valley-fill alluvium, areally extensive alluvium, glacial deposits, and basin-fill deposits. The aquifers normally consist of alluvial sand and gravel that contain unconfined ground water that lies near the land surface. Many wells completed in the alluvial aquifers have high yields of good-quality water because most alluvial aquifers are highly transmissive and hydraulically connected to streams. Aquifers in glacial deposits may be difficult to locate and in some areas contain saline water; nevertheless, these aquifers are sources of water supply for many users. Basin-fill aquifers are as much as several thousand feet thick, and many contain large ground-water supplies. Ground-water mining has occurred in semiconsolidated aquifers because of withdrawals from wells. Unconsolidated and semiconsolidated aquifers have potential for conjunctive use with surface water, recycling to reuse available supplies, artificial recharge, and salvage of evapotranspired water.

Sandstone aquifers lie near the land surface and in structural basins. Interbasin movement of ground water occurs in the Virgelle (Milk River aquifer), Fox Hills-basal Hell Creek, and Dakota aquifers. Sandstone aquifers are less transmissive than unconsolidated and semiconsolidated aquifers in general. Confined sandstone aquifers are common, and flowing wells are obtained in many areas. However, flowing wells may cause large declines in water levels if uncontrolled. Water quality is variable in sandstone aquifers but is adequate for most needs. Sandstone aquifers have potential for artificial recharge, induced interaquifer leakage, conjunctive use with surface water, and mining of ground water.

Limestone and dolomite aquifers are extensive in the region, but in some areas they lie deep below the land surface. The occurrence of ground water in small pores, fractures, or large caverns causes yields from wells to range widely. Large flows through cavern systems in the aquifers are indicated by large springs in some areas. Water quality is extremely variable and must be considered in any water-development plan. Limestone and dolomite aquifers have potential for development of large water supplies in some areas. The development may be aided by induced recharge and interaquifer leakage.

Saline ground water occurs throughout the Missouri Basin Region. Dissolved-solids concentration as much as 30,000 milligrams per liter has been measured in aquifers in glacial deposits in Montana. Saline water is common in sandstone aquifers in Wyoming, North Dakota, and South Dakota; maximum reported concentration is 280,000 milligrams per liter in water from the Tensleep Sandstone in Wyoming. Limestone contains saline water in many areas; maximum dissolved-solids concentration is about 350,000 milligrams per liter for the Madison Group in the Williston Basin in North Dakota.

Comprehensive water-management planning in the Missouri Basin Region will require periodic or continuing inventory of precipitation, streamflow, surface-water storage, and ground water. Water demands for irrigation, industrial, public supply, and rural use are increasing rapidly. Reliance on ground-water supplies is increasing even though in many areas the ground water is still mostly undeveloped. Optimal use of water supplies will require the establishment of realistic goals and carefully conceived water-management plans, each of which will necessarily be based on an adequate baseline of hydrologic data and knowledge of the highly variable hydrologic systems in the region.