| Citation |
Mills, T.J., Schweiger, E.W., Mast, M.A., Clow, D.W., 2012, Hydrologic, water-quality, and biological characteristics of the North Fork Flathead River, Montana, water years 2007–2008: U.S. Geological Survey Scientific Investigations Report 2011–5221, 67 p. |
| Abstract/Description or Keywords |
In water year 2007, the U.S. Geological Survey (USGS), in cooperation with the National Park Service (NPS), began a 2-year study to collect hydrologic, water-quality, and biological data to provide a baseline characterization of the North Fork Flathead River from the United States-Canada border to its confluence with the Middle Fork of the Flathead River near Columbia Falls, Mont. Although mining in the Canadian portion of the North Fork Basin was banned in 2010 by a Memorandum of Understanding issued by the Province of British Columbia, baseline characterization was deemed important for the evaluation of any potential future changes in hydrology, water quality, or aquatic biology in the basin. The North Fork Basin above Columbia Falls (including Canada) drains an area of 1,564 square miles, and the study area encompasses the portion of the basin in Montana, which is 1,126 square miles. Seasonal patterns in the hydrology of the North Fork are dominated by the accumulation and melting of seasonal snowpack in the basin. Low-flow conditions occurred during the late-summer, fall, and winter months, and high-flow conditions coincided with the spring snowmelt. Substantial gains in streamflow occurred along the study reach of the North Fork, 85 percent of which were accounted for by tributary inflows during low-flow conditions, indicating unmeasured streamflow inputs along the main stem were 15 percent or less. Specific conductance exhibited an inverse relation to streamflow in the North Fork due to the influx of dilute snowmelt water during high-flow conditions. Additionally, median specific conductance during the study was greater at the Flathead River at Flathead British Columbia gaging station (USGS station 12355000; referred to in this report as “Border”) than at the Columbia Falls station (USGS station 12355500; referred to in this report as “Columbia Falls”) due to dilution in the main stem by low-conductivity tributary inputs south of Border. Similar to specific conductance, concentrations of major ions had an inverse relation to streamflow and generally decreased in the North Fork in a downstream direction between Border and Columbia Falls. Additionally, water-quality data collected during the study generally indicate that calcium, magnesium, and alkalinity are the dominant solutes in the North Fork. Nutrient (nitrogen and phosphorous compounds) concentrations exhibited a different seasonal pattern than that of major ions, reflecting the differences in sources. Total nitrogen and total phosphorus (dissolved plus particulate) concentrations at Border and Columbia Falls were near or below their respective analytical reporting levels during the winter months but increased during spring snowmelt. The increased total nitrogen concentrations during spring snowmelt are likely not related to anthropogenic activities in the basin; rather, they are attributed to atmospheric deposition of nitrate during the winter months and subsequent release from the snowpack in the spring. Total phosphorous concentrations varied with the amount of suspended sediment in the stream, which increases substantially with streamflow in the North Fork. Trace-element concentrations at Border and Columbia Falls were near or below their respective analytical reporting levels for much of the year, but became elevated during high-flow conditions. Because trace-element samples were unfiltered, the elevated concentrations during snowmelt are likely associated with increased concentrations of suspended sediment during this time. Generally, variability in loading of most major ions among tributaries corresponded with variations in streamflow. However, tributary sulfate loads varied with concentration, reflecting the presence of an unidentified sulfate source in the northern tributaries. Trend analysis was performed on data from Border and Columbia Falls and one additional long-term water-quality site on the North Fork. A significant, flow-adjusted, upward trend in specific conductance was detected at Columbia Falls for 1982–2008. Additionally, a significant flow-adjusted upward trend in concentrations of alkalinity was detected at the Flathead River QW site (a long-term water-quality site operated by Environment, Canada located north of the International Border) for 1979–1995, and significant flow-adjusted upward trends in, calcium and magnesium were detected at Border for 1974–1993. The magnitude of these trends is small, but it could be indicative of increasing weathering or erosion in the basin. Diatom and macroinvertebrate samples were collected at Border and Columbia Falls. Despite elevated nutrient and lead concentrations during high-flow conditions, biological metrics do not indicate habitat impairment. However, some samples did show signs of sediment impairment, but assemblage composition was largely intact and generally indicative of high-quality aquatic habitat. Further characterization of biologic assemblages in the region will likely aid in the development of bioassessment metrics specific to the North Fork. |
