The U.S. Geological Survey and the Wyoming Department of Environmental Quality are involved in a cooperative effort to assess the probable cumulative impacts of coal mining on the hydrology of a part of the Powder River Structural Basin in Wyoming. It was assumed that the principal impacts on the ground-water system due to mining will occur in the relatively shallow aquifers which can be grouped into three homogeneous aquifers, namely, the Wyodak coal, the overburden, and the under burden. Emphasis of this report is on the results of analysis of surface-water resources in the Caballo Creek drainage. A surface-water model of the Caballo Creek drainage was developed using the Hydrological Simulation Program-Fortran model to help assess the impacts of mining activities on streamflow. The Caballo Creek drainage was divided into 10 land segments and 6 stream reaches in the modeling process. Three simulation runs show little, if any, change in streamflow between pre- and post-mining conditions and very little change between pre-mining and during-mining conditions. The principal reason for the absence of change is the high infiltration rate used in the model for all three conditions.
