Since 1974 a network of water-quality stations has been operated in the coal area of southeastern Montana. This report updates a previous report with 2 years of additional data collection and presents statistics and regression equations for water-quality variables. The most apparent feature of the study is the variability of water quality. Time-trend differences are most noticeable, with areal differences being present but more subtle. In comparing stations at the mouths of the five major drainages entering the Yellowstone River from the study area, water from the Powder River ranks near the middle of the group in dissolved-solids concentration (mean of 1,390 mg/L), but far exceeds the other drainages in suspended-sediment concentration, often exceeding 10,000 mg/L. The Tongue River generally has the best overall quality with respect to dissolved constituents; extremes are moderated by mixing in the Tongue River Reservoir. Suspended sediment ranged from 5 to 4,360 mg/L. Rosebud Creek shows about a 50-percent average increase in dissolved-solids concentration from the most upstream station to the mouth. Armells and Sarpy Creeks, smallest of the five drainages, have a pool-riffle configuration that influences both dissolved and suspended constituents. Pools permit greater evaporation, thus increasing dissolved-constituent concentrations. They also act as sediment traps. (USGS)
