river rising in N Texas in three forks; the Clear Fork runs into the West Fork at Fort Worth, and the Elm Fork joins the West Fork at Dallas. The Trinity then flows c.510 mi (820 km) SE to Trinity Bay, an arm of Galveston Bay. The waters of upper tributaries and the main stream are impounded in numerous reservoirs that provide water for the Dallas–Fort Worth metropolitan area; flood control; and water for irrigation. The largest reservoir, Garza–Little Elm, is impounded by Lewisville Dam (completed 1955) on the Elm Fork. The Trinity valley has a greater population and the majority of industrial development as opposed to other river basins in Texas. Massive flooding of the river occurred in the spring of 1990, recorded as among the nation's worst floods in the 20th cent.