The Hydrologic Simulation Program - Fortran (HSPF) was applied to the Illinois River Basin using the U.S. Environmental Protection Agency's (USEPA) Better Assessment Science Integrating Point and Nonpoint Sources (BASINS) system. Values of the HSPF model parameters were based on the calibrations of three representative watersheds within the basin. Over the 1985-1995 simulation period, monthly and annual mass balances correlated well with observed discharges at three gaging stations along the Illinois River. However, poor correlation of daily flows was due largely to the weakness of the HSPF model in routing dynamic flows through the complicated Illinois River system. To address this problem, a one-dimensional unsteady state flow model (UNET) developed for the main stem of the Illinois River was coupled with the HSPF model to perform the flow routing. Comparison of hydrographs showed much closer correlation between coupled HSPF-UNET model simulations of daily flows and observed flow hydrographs. This study also showed that peak flows simulated by the HSPF model occurred several days before the actual observations. However, the coupled HSPF-UNET model simulated peak flow time more closely to the observations. The Nash-Sutcliffe efficiency (NSE) computed using observed and simulated daily flows from HSPF and HSPF-UNET models was 0.72 and 0.79 at Marseilles, 0.63 and 0.82 at Kingston Mines, and 0.40 and 0.85 at Valley City, respectively. As expected, the NSE computed using mean monthly flows was also slightly better from HSPF-UNET than from HSPF, but the annual NSE values were nearly same from both models.