The southern Powder River area comprises about 2,230 square miles in southeastern Montana. Sedimentary rocks beneath this area range in age from Cambrian to Holocene and attain a maximum thickness of about 12,000 feet. Rocks exposed in the study area range in age from Late Cretaceous to Holocene.

Water for domestic, stock, and public use in the southern Powder River area is available from the Fox Hills-lower Hell Creek aquifer and the upper part of the Hell Creek Formation, both of Late Cretaceous age, and the Fort Union Formation of Paleocene age. Water for industrial use might be available from the Madison Group of Mississippian age. Where adequate quantities are available, water from the Madison Group is suitable for irrigation. Water from alluvium of Holocene and Pleistocene age along the Powder River is marginally suitable for irrigation, and surface water from the Powder River is suitable for irrigation, except during periods of low flow.

The Madison Group is a sequence of carbonate rocks that underlies the entire study area and ranges in thickness from 800 to 1,500 feet. Depth to the top of the group is about 4,000 feet in the southeastern part of the area and as much as 9,000 feet in the southwestern part. Water-supply wells at the Bell Creek oil field in the southeastern part of the study area flow as much as 1,300 gal/min (gallons per minute). Oil-field water from this area contains calcium, magnesium, and sulfate as major ions, and contains less than 1,000 mg/L (milligrams per liter) dissolved solids.

The Fox Hills-lower Hell Creek aquifer is predominantly sandstone interbedded in part with sandy shale and siltstone. The aquifer crops out in the eastern part of the study area and dips generally westward beneath the land surface at about 40 to 200 feet per mile. Wells in the Fox Hills-lower Hell Creek aquifer range in depth from 25 feet in the outcrop area to 1,358 feet between the Powder and Little Powder Rivers. Measured well yields range from 3 to 200 gal/min. Many flowing wells along the principal rivers yield as much as 20 gal/min. The water contains calcium, magnesium, and bicarbonate or sulfate as major ions in the outcrop area. Downgradient, sodium and bicarbonate are predominant. The dissolved-solids concentration ranges from 261 to 2,230 mg/L.

The upper part of the Hell Creek Formation consists of interbedded shale, siltstone, claystone, and sandstone. Wells in the upper part of the Hell Creek are as deep as 1,045 feet. Flowing wells along the Powder and Little Powder Rivers yield as much as 40 gal/min, but pumped wells generally yield less. The water quality is similar to that of the Fox Hills-lower Hell Creek aquifer. The dissolved-solids concentration ranges from 236 to 1,580 mg/L.

The Fort Union Formation consists of interbedded sandstone, siltstone, shale, and coal. The formation crops out in the western three-fourths of the study area and is about 2,100 feet thick in the southwestern part. Wells tapping this unit range in depth from 120 to 1,100 feet. Wells and springs are reported to yield as much as 20 gal/min; many wells flow. The upper part of the formation is a major aquifer west of the Powder River valley. The water contains calcium, magnesium, sodium, bicarbonate, and sulfate as the major ions in most any relative proportion. The dissolved-solids concentration ranges from 541 to 2,160 mg/L.

Alluvium is unconsolidated deposits of interbedded clay, silt, sand, and gravel, mostly along the river valleys. Maximum measured thicknesses of these deposits were 62 and 55 feet along the Powder and Little Powder Rivers, respectively. Several irrigation wells drilled in the alluvium along the Powder River have reported yields of as much as 900 gal/min. Water contains calcium, magnesium, and sulfate as major ions. The dissolved-solids concentration of two water samples from the Powder River valley was 1,780 and 2,240 mg/L.

The only perennial streams are the Powder and Little Powder Rivers. Based on available streamflow records, the mean-annual discharge of the Powder River at Moorhead, in the southwestern part of the area, is 448 cubic feet per second; discharge varies from 0 to 23,000 cubic feet per second. Mean-annual discharge of the Little Powder River near Broadus, in the north-central part of the area, is 31 cubic feet per second; discharge varies from 0 to 2,440 cubic feet per second. Water from the Powder River normally contains calcium and sulfate as the major ions. Dissolved-solids concentration ranges from 20 to 3,460 mg/L.