Hydrologric data were used to determine the premining surface-water conditions in two small basins in the Fort Union coal region of Montana and North Dakota. The two streams. Hay Creek and West Branch Antelope Creek, are ephemeral. Most of the volume and peak discharges are due to snowmelt runoff. Little rainfall runoff occurs, and volume and peak discharges for this runoff are relatively small compared to those for snowmelt runoff
Suspended-sediment concentrations for snowmelt runoff ranged from 4 to 325 milligrams per liter for the Hay Creek and West Branch Antelope Creek watersheds. At the outflow site of the Hay Creek watershed, the dominant dissolved constituents in runoff are magnesium and sulfate; at the outflow site of the West Branch Antelope Creek watershed, they are calcium, magnesium, bicarbonate, and sulfate.
The U.S. Geological Survey's Precipitation-Runoff Modeling System was calibrated for both watersheds for the snowmelt runoff. The model was not calibrated for rainfall runoff because of insufficient runoff. Sensitivity analyses indicated the model was most sensitive to the values of snow correction for daily precipitation at precipitation gages, emissivity of the air for longwave radiation, and maximum available water-holding capacity of the soil profile. Testing of several watershed delineations showed that, for well-defined snow distribution, 23 units adequately defined the variability in runoff in the Hay Creek watershed, and 36 units adequately defined the variability in runoff in the West Branch Antelope Creek watershed.
