Guadalquivir (ancient Baetis), is rising in the Sierra de Cazorla in SE Spain. It flows 602 km (374 mi), generally southwest, past Cordoba and Seville to Sanlucar de Barrameda, where it empties into the Gulf of Cadiz, an arm of the Atlantic Ocean. In its middle course it flows through a populous fertile region at the foot of the Sierra Morena, where it is used extensively for irrigation. The area has a rich variety of plant life. For the 64 km (40 mi) between the town of Coria del Río and the mouth of the river, the Guadalquivir traverses a region of tidal marshes called Las Marismas. Supplied by rainwater in the winter and by the melting snows of the Sierra Nevada in the summer, the river maintains a full flow throughout the year, providing essential irrigation water and hydroelectric power. During the Moorish occupation of Spain in the Middle Ages, the river was navigable as far as Córdoba; now, as a result of silt accumulation, it is navigable no farther upstream than the city of Seville, a distance of about 80 km (50 mi). The river's name derives from the Arabic Wadi al-Kabir ("Great River"), the name given to it by the Moorish rulers of Córdoba from the 8th century onwards.