The main rivers of Spain flow west and southwest to empty into the Atlantic Ocean; usually they run through deep and rocky valleys through mountain courses. These rivers include the Douro, the Minho, Tagus and Guadiana born in Spanish territory and flow through Portugal, or constitute the border with this country before emptying into the Atlantic. The Guadalquivir, which crosses a fertile plain in the south, is the only navigable river in Spain, but only for shallow draft boats in its last 100 km, from Seville to its mouth. The Ebro, the largest river in Spain, takes the opposite direction northwest-southeast, and belongs to the Mediterranean side. Most Spaniards are little mighty rivers and therefore not suitable for inland navigation, although widely used for irrigation and in its upper and middle classes, have an important advantage as an energy source.