Valley-Fill Hydrology and Groundwater Withdrawals, Fort Belknap Indian Reservation: 1993 Summary

Description

The report investigates the hydrology of valley-fill aquifers and assesses the potential for additional groundwater withdrawals for irrigation, stock, and domestic use along the north flank of the Little Rocky Mountains. The study area includes five principal stream valleys: Little Peoples, Lodge Pole, Jim Brown, Big Warm, and Beaver Creeks.

Findings

• Valley fill consists of cobbles, gravel, sand, silt, and clay deposited as glacial deposits, colluvium, and alluvium. Glacial deposits are relatively impermeable and often form a confining unit overlying the valley-fill aquifer. Alluvium, up to 90 feet thick near the mountain front, contains sand and gravel layers forming the primary aquifer.
• Little Peoples Creek & Lodge Pole Creek: Only these valleys have continuous sand and gravel deposits along most of their length. Aquifers are unconfined in southern, upgradient, unglaciated parts and confined in northern, downgradient parts (overlain by glacial sediments). Hydraulic conductivity decreases with distance from the mountain front (max: 760 ft/day near mountains, min: 180 ft/day downgradient).
• Aquifers are recharged by infiltration of streamflow, runoff, precipitation, and leakage from bedrock. Discharge occurs via leakage to streams, evapotranspiration, and outflow through valley fill at the downstream end. Water levels in unglaciated areas respond quickly to streamflow; confined (glaciated) areas show little response.
• Water near the mountain front is calcium bicarbonate type with low dissolved solids (as low as 232 mg/L). Downgradient water is sodium sulfate or magnesium sulfate type, with dissolved solids up to 11,500 mg/L. Increased dissolved solids result from bedrock leakage, mineral dissolution, and evapotranspiration. Some wells exceed drinking water standards for dissolved solids, sulfate, iron, manganese, and cadmium.
• Little Peoples Creek: Good potential for additional withdrawals. Large-capacity wells (150–250 gal/min) for irrigation could lower water levels, increase bedrock leakage, and increase infiltration from the creek, possibly raising dissolved solids and reducing streamflow.
• Lodge Pole Creek: Limited potential for large-capacity wells due to insufficient recharge. Southern, unconfined aquifer suitable for domestic and stock-watering use; northern, confined aquifer may have undesirable water quality.
• Jim Brown, Big Warm, Beaver Creeks: Insufficient aquifer thickness or recharge for large-capacity irrigation wells. Beaver Creek near the mountain front may support small-scale domestic and stock wells.
• Valley-fill aquifers are bounded by shale except where valleys cross permeable sandstones. Aquifer geometry and hydraulic properties vary between valleys and along their length. Additional withdrawals are feasible in certain areas but may impact water quality and streamflow. Water quality issues (high dissolved solids, sulfate, iron, manganese) may limit domestic use in some areas.