Human alteration of waterways has impacted the minimum and maximum streamflow in more than 86% of monitored streams nationally and may be the primary cause for ecological impairment in river and stream ecosystems. Restoration of freshwater inflows can positively affect shellfish, fisheries, habitat, and water quality in streams, rivers, and estuaries. Increasingly, state and local decision makers and Federal agencies are turning their attention to the restoration of flows as part of a holistic approach to restoring water quality and habitat and protecting and replenishing living coastal and marine resources and the livelihoods that depend on them. In 2017, the U.S. Geological Survey (USGS) Lower Mississippi-Gulf Water Science Center began developing datasets, methods, and tools to support a two-phase, seven-year study of freshwater delivery and magnitude in the five Gulf States (Alabama, Florida, Louisiana, Mississippi, and Texas). This study is in collaboration with the U.S. Environmental Protection Agency and funded through the Resouces and Ecosystems, Sustainability, Tourist, Opportunities, and Revived Economies of the Gulf Coast States Act (RESTORE Act). The first phase of this study is focused on (1) quantification of hydrologic alteration, (2) evaluation of trends in the delivery and magnitude of freshwater to estuaries, and (3) analysis of the current streamgage network suitability for the five Gulf States. The second phase focuses on a large watershed in Mississippi and will address (1) evaluation and identification of flow-ecology relationships and (2) development of a decision-support tool that integrates existing streamflow goals and constraints as well as determined flow-ecology relationships. This dataset represents a boundary delineating the spatial extent of the study. USGS streamgages located within the boundary either drain to the Gulf of Mexico or are adjacent to watersheds that flow to the Gulf of Mexico and are considered both physiographically similar and valuable for analysis. The boundary extent was delineated using geographic information system (GIS) tools to represent the geographic extent of the area important in analyzing streamflow alteration. The area within the boundary is 1,004,220 square kilometers and contains 9,966 12-digit hydrologic unit code (HUC) polygons and 294 8-digit HUC polygons.