The U.S. Geological Survey (USGS) Lower Mississippi-Gulf Water Science Center (LMGWSC) in collaboration with the U.S. Environmental Protection Agency and funded through the Resources and Ecosystems, Sustainability, Tourist, Opportunities, and Revived Economies of the Gulf Coast States Act (RESTORE Act) are conducting a multiyear multistate study to analyze the alteration and trends of streamflow delivery to the Gulf of Mexico; the results of of which are used in an OASIS model decision support tool (Hazen and Sawyer, New York, NY, USA). To fully develop the OASIS model, a geospatial polyline boundary delineating the Mobile and Perdido bays OASIS streamflow prediction model study area was created. The spatial extent of this polyline includes the upstream contributing drainage area of Mobile and Perdido bays, encompassing Mobile-Tombigbee (HUC04 0316), Alabama (HUC04 0315), Perdido (HUC08 03140107), and Perdido Bay (HUC08 03140106) watersheds. Another geospatial polyline is provided and is a buffer around the study area. The buffer is bounded by the drainage divide of the Mississippi River to the west, the drainage divide of the Apalachicola-Chattahoochee-Flint River System to the east, and the drainage divide of the Tennessee River to the north. The purpose of the buffer is to provide a geographic extent larger than the study area that can be used to filter data used in regional modeling. Having training data adjacent to the study area can limit error associated with regional modeling at the geographic edges of the study area. The statistical buffer spatial extent was delineated using well-known geographic information system tools and the National Hydrography Dataset Hydrologic Unit Code geospatial files and other infrastructure.