The land surface (topography) of the Sweet Grass Hills and the surrounding area is largely a product of geologic forces. Igneous intrusive rocks caused the uplift of the hills followed by erosion of these uplifted units. The more resistant limestone and igneous rocks form steep-sided buttes. Less resistant sandstones and shales form subdued foothills encircling the buttes. Igneous dikes and sills form irregular ridges and knobs. Below about 5,000 feet in elevation, glacial deposits form an undulating surface which becomes less evident as the foothills melt into the surrounding prairies.
The climate of the Sweet Grass Hills is semiarid. The orographic effect of the Sweet Grass Hills results in average annual precipitation reaching 20 inches on the highest buttes and dropping to 10 to 12 inches at Chester, 20 miles south of the Sweet Grass Hills. May through August is generally the wettest period of the year with approximately 65 percent of the annual precipitation occurring in this period. The mean annual temperature is approximately 42 degrees Fahrenheit (F) with extremes ranging from 100 degrees F for short periods during the summer to -50 degrees F during severe winters. The average growing season is about 90 days.
The streams in the Sweet Grass Hills are ephemeral or intermittent, although short segments of some of the larger streams tend to be perennial. Flow results from snowmelt or intense summer storms. Snow accumulated throughout the winter in dense drifts allows some streams to flow into mid summer. Streams on the north side of the Sweet Grass Hills flow into the Milk River (north into Alberta, Canada) while streams flowing from the south side flow into the Marias River. Streams tributary to the Milk River are classified B-1 by the Montana Water Quality Act (MDHES, 1982) while streams tributary to the Marias River are classified B-2. Water quality is excellent in the upper portions of the streams, but decreases in the downstream direction as it picks up sediment on the plains and ground water contributions from underlying formations. The water is a sodium to calcium bicarbonate type and slightly alkaline. Specific conductance (SC) ranges from 340 to 1,170 microsiemens per centimeter (uS/cm), pH values range from 7.7 to 8.5, and total dissolved solids (TDS) concentration ranges from 240 to 780 milligrams per liter (mg/l) (USGS, 1993).
Little data is available to describe the ground water hydrology in the higher elevations in the Sweet Grass Hills. More data is available for the foot hills and plains within the study area. Doming, fracturing, faulting, and folding greatly influence the occurrence and movement of ground water in the study area. Surface geologic data suggests shallow ground water (within 500' of the surface) movement within the Sweet Grass Hills generally follows the surface topography. The ground water is recharged locally by surface flows over exposed limestones, sandstones, and glacial deposits of sand and gravels on the flanks of the Sweet Grass Hills. In the foothills and plains the aquifer systems become more continuous. Two principal unconsolidated aquifer systems are the shallow alluvium of the principal streams, and the interstratified sand gravel in glacial deposits. Two consolidated aquifer systems also exist in the study area, the Judith River Formation and the Virgelle Sandstone Member of the Eagle Sandstone.
These four aquifer systems are sources for the 90 springs and 40 wells in the study area. Approximately one-half of these springs and wells are used for domestic purposes, while the remainder are used for stock water. One well, the Sage Creek Water Users well, serves over 60 users with an extensive pipeline system. Waters within these four aquifer systems flow into the regional ground water system and flow is generally to the north, northeast and east.
The United States Geological Survey (USGS) recently published a study of these four aquifer systems on the north flank of the Sweet Grass Hills describing water quality, quantity, and flow characteristics (USGS, 1993).
Water quality sampling from the alluvial aquifer at the Sage Creek and Bear Gulch sites, indicate generally good water quality. The water is a calcium bicarbonate type, with a pH of 7.1, SC of 670 uS/cm, and TDS of 439 mg/l (USGS, 1993). Water quality from the interstratified sand and gravel glacial deposits exhibited more variation in water quality. The water was generally suitable for stock watering, but varied from suitable to marginal to unsuitable for domestic use (USGS, 1993). One sample collected from the Judith River aquifer indicates water quality is marginal for domestic use, but suitable for stock water (USGS, 1993). Water quality from the Virgelle Sandstone is variable ranging from suitable to unsuitable for domestic purposes but generally suitable for stock use. This water is a calcium to sodium bicarbonate type with pH ranging from 7.2 to 8.6, SC from 392 to 2,070 uS/cm, and TDS from 213 to 1,360 mg/l (USGS, 1993).
Residents in or near the study area are highly dependant upon domestic water either wholly or partially derived from the Sweet Grass Hills. Water of suitable domestic quality either becomes deficient in quality, quantity, or too deep for economical recovery in directions away from the Sweet Grass Hills.
A current hydrologic study is concentrating on water resources of the East Butte portion of the Sweet Grass Hills. The Montana Bureau of Mines and Geology (MBMG) is conducting this study with a scheduled completion date of April, 1996. Conversations with the MBMG, based on preliminary information, indicate that the area is sensitive to surface disturbing activities such as mining and oil and gas. Any of these activities has the potential to impact the shallow aquifer system surrounding East Butte. This is consistent with the analysis in the amendment/EIS which is based on information from the United States Geological Survey (USGS, 1993) on the four aquifer systems in the Sweet Grass Hills. The additional information specific to East Butte is not essential for a reasoned choice among the alternatives. This additional information may be useful for evaluating Manhattan Minerals (US) Ltd. pending exploration proposal in the East Butte area once the BLM continues processing of the proposal.
Presently no water rights claims are filed on public lands within the study area except the implied instream stock use. The Montana Department of Natural Resources and Conservation has imposed a basin closure to Breed, Miner's, and Bear Gulch Creeks and their tributaries for future consumptive development of surface water. An exception was granted for livestock and domestic developments which use less than three acre feet per year. Applications for consumptive use of surface waters in these drainages for irrigation, mining, or other uses are not currently being accepted. Other drainages in the study area remain open for applications for both surface and ground water uses.