The Bays of the Texas Coastal Bend encompass 3 distinct estuarial or bay regions. To the north, the Mission-Aransas Estuary System receives inflow from the Mission and Aransas rivers. This estuary includes Aransas Bay, Copano Bay, Port Bay, St. Charles Bay, and Mesquite Bay.
The middle section of the Texas Coastal Bend Bay system is influenced by the Nueces River Estuary System. The Nueces estuary system includes several distinct areas including Corpus Christi Bay, Oso Bay, Nueces Bay, Redfish Bay, and the bay sides of Harbor Island, and Mustang Island.
The southern section of the Texas Coastal Bend is less influenced by estuarial waters and is a unique system of tidal flows in the Upper Laguna Madre and the hyper-saline waters of Baffin Bay. Baffin Bay was originally a valley formed by a small river system that drained into the Gulf of Mexico. Through prehistoric periods, the sea levels rose, flooding the river valley and Padre Island formed as a barrier, effectively cutting off the area from the Gulf.