The Tagus ( ; ; ; see below) is the longest river in the Iberian Peninsula. The river rises in the Montes Universales in mid-eastern Spain, flows , generally west with two main south-westward sections, to empty into the Atlantic Ocean near Lisbon. Its drainage basin covers – exceeded in the peninsula only by the Douro. The river is highly used. Several dams and diversions supply drinking water to key population centres of central Spain and Portugal; dozens of hydroelectric stations create power. Between dams it follows a very constricted course, but after Almourol, Portugal it has a wide alluvial valley, prone to flooding. Its mouth is a large estuary culminating at the major port, and Portuguese capital, Lisbon.