Texas has 367 miles of coastline within which 21 river basins terminate, bringing fresh water from surface run-off, rivers, and streams into individual bays. Texas has seven major and five minor estuaries ranging from the nearly fresh Sabine Lake, which borders Louisiana, to the frequently hypersaline Laguna Madre along the southern coast. Although each estuary differs in size, hydrologic, and ecological characteristics, together they support a diverse array of species which serve as the raw materials for a variety of economic activities associated with commercial and recreational fishing, hunting, and birding. In addition, they provide many other ecological services such as water filtration, nutrient regulation, storm surge protection, and shoreline stabilization.

The quality, quantity, and timing of freshwater inflow from rivers is important to maintaining the natural salinity, nutrient, and sediment loading regimes which support the unique biological communities of each estuary and also to ensure healthy ecosystem function. As the state's population grows, increasing demands for water may limit the volume of freshwater reaching the bays as well as alter the quality or timing in which the water arrives, thus impacting the productivity and characteristic of Texas estuaries. The TWDB's Bays & Estuaries Program mission is to maintain a continuous data collection, modeling, and analytical study program focused on evaluating the freshwater inflow requirements necessary to maintain the health of Texas bays and estuaries.

The Texas Water Development Board has continuously monitored estuarine salinity and water quality since 1986. Data collection activities for the Estuary Monitoring Program are conducted by the TWDB to maintain a data collection and analytical study program focused on determining the effects of and needs for freshwater inflows to the state's bays and estuaries. The purpose of such monitoring is to support calibration and validation of estuarine hydrodynamic and salinity transport models and for development of freshwater inflow-salinity relationships to aid in water resources planning.