Potentiometric-surface map of water in the Eagle Sandstone and equivalent units in the northern Great Plains area of Montana

The potentiometric surface of water in the Upper Cretaceous Eagle Sandstone and equivalent units (Virgelle Sandstone and Telegraph Creek Formation) is shown on a base map at a scale of 1:1,000,000. The map is one of a series of maps produced as part of a regional study of aquifers of Cenozoic and Mesozoic age in the northern Great plains of Montana. The contour interval is 200 feet. The map shows that the directions of regional ground-water movement is from west to east except in the extreme northwestern corner where the movement is from east to west. Water is discharged from the Eagle Sandstone to the Missouri River south of the Bearpaw Mountains. In the area north of the Bearpaw Mountains, depressurization from gas production may account for the decline in the potentiometric surface. The average discharge from 115 wells is about 23 gallons per minute and the specific capacity of 85 wells averages 1.4 gallons per minute per foot of drawdown.