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. The Houston Ship Channel passes through Bolivar Roads, Galveston Bay, the San Jacinto River, and Buffalo Bayou to the Port of Houston, which in terms of tonnage was the third largest United States port during the 1980s. At that time around 4,700 ships traversed Galveston Bay each year to and from its principal ports: Galveston, Texas City, and Houston. Also in the 1980s, three-quarters of the Texas coastal population lived in the counties bordering Galveston Bay, and 29 percent of all marinas on the Texas coast were on the bay. Between 1817 and 1820 Galveston Bay provided the water connection for the illicit smuggling and privateering activities of the Laffite brothers (see LAFFITE, JEAN). During the Civil War, the battle of Galveston was fought on land, along the wharf area, and on the waters of the bay in December 1862 and January 1863.