EXTENT AND HYDROGEOLOGY OF THE LOWER YELLOWSTONE BURIED CHANNEL AQUIFER SYSTEM, RICHLAND COUNTY, MONTANA

A buried-channel aquifer capable of producing large quantities of good-quality water, suitable for most uses, underlies the Yellowstone River valley in eastern Richland County, Montana. The aquifer is composed of up to 100 feet of saturated sand and gravel that appears to be glacial outwash infilling a 1.5- to 2-mile wide valley incised into Fort Union Formation bedrock on the southwest flank of the Williston Basin. The aquifer was initially identified in the 1960s beneath terraces near the City of Sidney for which it provides municipal water. More recent work by the Montana Bureau of Mines and Geology (MBMG) has traced the aquifer to the North Dakota border, where it underlies the modern Yellowstone River valley. Preliminary work to determine the southern extent of the aquifer shows that it underlies high terraces of up to 300 feet above the present day Yellowstone River between locations south of Fox Creek upstream to Burns Creek.

Because there is little or no surface expression of this buried channel aquifer system, the aquifer has been mapped by compiling lithologic logs of existing water wells and information gathered from several small-scale drilling projects completed during 2006, 2007, and 2013. Yellowstone River tributaries typically change from losing streams, as they cross the western upstream boundary of the aquifer, to gaining streams as they approach its eastern downstream edge. The aquifer’s eastern boundary can often be defined by locating these changes in surface-water discharge.

The potential capacity of this aquifer is promising, especially for irrigating the overlying dry-land valley slopes and terraces. Additionally, recent energy development in western North Dakota and eastern Montana has increased local demand for large-volume water sources and aquifer tests near Crane Creek recently verified that the aquifer can support properly constructed wells capable of producing from 800 to 1,000 gallons per minute. Discharges of this magnitude are commonly needed to support irrigation development, municipal supplies, and large scale industry. Recharge-discharge relationships, maximum pumping rates, and potential impacts to other water resources are being investigated by MBMG to help the Richland County Conservation District and local landowners determine the aquifer’s development potential.