Estuaries are defined by mixing of river and sea water, thus freshwater inflow is a key driver of estuary ecosystem structure and function. While there is much concern about water quality, there is much less about water quantity. As water is diverted for human use, less is flowing to the coast, which threatens estuary ecosystems. Some jurisdictions are now setting inflow standards, but there is no consensus on how to identify how much freshwater an estuary needs. There is a climatic gradient along the northwestern Gulf of Mexico coast and estuaries vary from hydrologically positive to neutral to negative, and this makes the Texas coast the ideal place to study how ecological processes vary with freshwater inflow. An estuary comparison approach is used in this open access work to examine hydrology, circulation, salinity, nutrients, carbonate, dissolved oxygen, plankton, nekton, benthos, and habitat dynamics and responses across varying hydrological regimes.