GALVESTON BAY. Galveston Bay, the center point of which is 29°34' north latitude and 94°56' west longitude, is part of Harris, Galveston, and Chambers counties. It is the largest estuary on the Texas coast and the seventh largest in the United States. It extends thirty miles south to north and seventeen miles east to west. The bay is seven to nine feet deep. It has a mud bottom and about 600 square miles of surface. Fresh water from the Trinity and the San Jacinto rivers mixes with the tidal salt water from the Gulf of Mexico through the channel between Galveston Island and the Bolivar Peninsula (Bolivar Roads). During the 1980s the bay provided nursery and spawning grounds for 30 percent of the state's total fishing products.