Southern Vietnam is dominated by the rich agricultural lands of the Mekong Delta; the nation's major source of rice; fruit and vegetable production. The land is alluvial and low-lying; with marshlands and mangrove forests to the west and south. During the rainy season; water covers a third of the delta; with flooding up to 13 ft (4 m). In places; salt water encroaches into the delta for up to 30 m (48 km). Although extraordinarily fertile; parts of the Delta have yet to come under cultivation.