Saint Louis Bay Watershed is a coastal watershed located in southern Mississippi, which composes two basins: Jordan River Basin and Wolf River Basin. Two land use/land cover (LULC) datasets: the Geographic Information Retrieval and Analysis System (GIRAS) and the National Land Cover Data (NLCD) are applied on these two basins to simulate the hydrologic and water quality parameters by using the Hydrological Simulation Program - FORTRAN (HSPF) and the Better Assessment Science Integrating Point and Nonpoint Sources (BASINS). The difference of GIRAS dataset and NLCD dataset for the Saint Louis Bay represents the LULC change from 1977 to 1992. The area percentages of Forest Land and Urban or Built-up Land decrease for both these two basins. However, the area percentages of Wetlands and Barren Land increase. The Rate of Soil Erosion has a great increase in the area changing from other LULC classifications to Barren Land. As a result of the loss of vegetation and the aggravation of soil erosion, the Total Outflows of Sediments has a great increase, which deteriorates the water quality. However, other hydrologic and water quality parameters including Streamflow, Water Temperature, and Dissolved Oxygen, in contrast, result in insignificant changes.