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A ground‐water flow system in southwestern Minnesota illustrates water movement between geologic units and between the land surface and the subsurface. The flow patterns indicate numerous zones of ground‐water recharge and discharge controlled by topography, varying thicknesses of geologic units, variation in permeabilities, and the configuration of the basement rock surface. Variations in streamflow along a reach of the Yellow Medicine River agree with the subsurface flow system. Increases and decreases in runoff per square mile correspond, apparently, to ground‐water discharge and recharge zones. Ground‐water quality variations between calcium sulfate waters typical of the Quaternary drift and sodium chloride waters typical of the Cretaceous rocks are caused by mixing of the two water types. The zones of mixing are in agreement with ground‐water flow patterns along the hydrologic section.

FIGURE 1. Map of area of this report _______ _______________ __ ____________________ J3 2. Diagrams showing well and test-hole numbering system________._ 3-10. Maps showing: 3. Generalized surficial geology and thickness of glacial drift-_____-__-_-_-_-_-_-____-___-___ _ _______________ 6 4. The configuration of the till surface underlying the surficial outwash west of Lake Emily ___.. _._ _ 8 5. Depth to the water table in surficial acquifer __ ___________ 11 6. Saturated thickness of the surficial aquifer _______ ________ 12 7. Transmissivity of the surficial aquifer _________.__ ____ 13 8. Configuration of water table and direction of groundwater flow in the surficial acquifer ________.. ___

The area of the proposed Cabin Creek coal mine was studied to obtain information needed to respond to questions posed by the International Joint Commission advisors concerning water resources near the international border. Specific interest focused on determining the extent and character of surficial material in the Flathead River valley, identifying gaining and losing reaches of the river and major tributaries, and documenting ambient water quality at selected sites.Quaternary alluvial streambed deposits consisting of clean sand, gravel, and boulders underlie the valleys of the Flathead River and major tributaries. Thickness of the alluvial deposits depends on depth to underlying Quaternary glacial deposits or Tertiary bedrock. The alluvial deposits in the Flathead River valley thin to a veneer of cobbles near the mouth of Couldrey Creek.Measurements of streamflow at 20 sites in the Flathead River valley indicate that water discharges from the alluvial deposits to most of the tributaries and to the river near the proposed mine. The Flathead River gained 0.87 cubic meter per second (31 cubic feet per second) of flow near Howell Creek. The Flathead River and Couldrey Creek gained about 0.81 cubic meter per second (28.5 cubic feet per second) of flow near the mouth of Couldrey Creek where bedrock crops out in the streambeds. Bedrock outcrops effectively interrupt the alluvial aquifer system between the proposed mine site and the international border. The Flathead River lost 0.87 cubic meter per second (31 cubic feet per second) of flow between the bedrock outcrops and the international border; this streamflow loss enters alluvial deposits and flows across the international border as subsurface flow.Analysis of samples from 18 stream sites and 1 spring site indicates general trends in water quality. In Howell Creek, concentrations of calcium, magnesium, and sulfate increased slightly downstream. Conversely, samples from Sage and Couldrey Creeks indicate downstream increases in concentrations of calcium, magnesium, and alkalinity, but decreases in concentrations of sulfate. Water quality of Cabin Creek was relatively stable through the sampled reach. Decreased concentrations of calcium and alkalinity in the Flathead River reflect the effect of inflow from Couldrey Creek and the ground-water inflow documented by streamflow measurements.

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