Ground water-surface water relations in the Flathead River valley near the proposed Cabin Creek coal mine, British Columbia, Canada

Abstract

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 advisers 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. Thickness of the alluvial deposits depends on depth to underlaying 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 gains 0.87 cu m/sec (31 cu ft/sec) of flow near Howell Creek. The Flathead River and Couldrey Creek gained about 0.81 cu m/sec (28.5 cu ft/sec) 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 cu m/sec (31 cu ft/sec) 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 sulfates 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 groundwater inflow documented by streamflow measurements. (Author 's abstract)