A new Surface-Water Routing (SWR1) Process was written for use with the U.S. Geological Survey (USGS) MODFLOW-2005 groundwater model. The SWR1 Process is designed to simulate surface-water routing in one- and two-dimensional surface-water features and surface-water/groundwater interactions. The performance of this computer program has been tested in models of hypothetical surface-water systems and integrated surface-water/groundwater systems; however, future applications of the programs may reveal errors that were not detected in the test simulations. Users are requested to notify the USGS if errors are found in the documentation report or in the computer program.