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The State of the Bay Galveston Bay Area Project

Water Quantity

 Estuaries are by definition systems in which freshwater from land and rivers mixes with salt water from the ocean. Freshwater enters Galveston Bay from a variety of sources; the Trinity and San Jacinto rivers, along with the bayou watersheds that flow directly into the bay, supply virtually all of the freshwater inflow to Galveston Bay. The amount, timing, location, and quality of freshwater entering the bay have long been issues for management, as human uses of freshwater (e.g., drinking water, irrigation, and industrial processes) affect the amount of water entering the bay, particularly during droughts, when freshwater is scarce. Complicating the matter is the extent of the Galveston Bay watershed. Water use by people as far away as Dallas–Fort Worth is directly tied to the amount of water flowing down the Trinity River to Galveston Bay.

The Trinity and San Jacinto Rivers are gauged for quantity of water carried to the bay. According to the Texas Water Development Board, on average the Trinity River contributes approximately 54 percent of annual freshwater inflow to Galveston Bay (Powell et al. 1997; Buzan et al. 2009). Much less is known about the hydrology of bayous, the most common type of tributary to Galveston Bay. Bayous operate primarily as extensions of the tidal bay system and may have no natural source of freshwater beyond precipitation.


Average monthly modeled freshwater inflows to Galveston Bay 1941-2009.  Data from Texas Water Development Board


The tributaries are not simply conduits for freshwater, nutrients, and sediment. They also contain dynamic ecological systems that process the materials. Without the inflow of freshwater, nutrients, and sediments transported by rivers and streams, the estuary would not exist. It would be a lagoon, a salty extension of the Gulf.  The Galveston Bay Freshwater Inflows Group (GBFIG) has been meeting since 1996 to devise strategies to maintain adequate freshwater inflows to Galveston Bay.   Read more about their efforts here.

Flooding occurs periodically in the Galveston Bay watershed and can happen in any of the tributaries. Flooding is a natural process connecting the aquatic and terrestrial ecosystems. It periodically introduces pulses of nutrient  and sediment-rich water necessary for the existence of floodplain forests and surrounding wetlands. Floods also bring dissolved and suspended organic material and contaminants to the estuary. Some flooding events in the Lower Galveston Bay watershed are driven by tides and storm surge rather than precipitation.


Literature Cited

Buzan, D., W. Lee, J. Culbertson, N. Kuhn, and L. Robinson. 2009. "Positive relationship between freshwater inflow and oyster abundance in Galveston Bay, Texas." Estuaries and Coasts no. 32 (1):206-212.

Powell, G. L., and R. S. Solis. 1997. Changes in freshwater inflows to Galveston Bay and the Trinity-San Jacinto Estuary, Texas  Paper read at Galveston Bay National Estuary Program, Proceedings, The State of the Bay Symposium III, pp. 11-12, 10-11 January 1997, at Galveston, Texas.

TWDB. 2001. Trinity-San Jacinto Estuary freshwater inflows (Galveston Bay): Contribution by basin. Texas Water Development Board 2001. Available from



Related Pages:

Water and Sediment
Water and Sediment Quality


Reservoir Map Four reservoirs have been created on major tributaries of the Galveston Bay system.Four reservoirs have been created on major tributaries of the Galveston Bay system.



Freshwater Inflows By Basin Five river basins supply Galveston Bay’s freshwater inflows. Data source (TWDB 2001).Five river basins supply Galveston Bay’s freshwater inflows. Data source (TWDB 2001).




Moses Lake floodgate. Image courtesy John Matthews.Moses Lake floodgate. Image courtesy John Matthews.

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