Szechwan Basin

(also Red Basin), an intermontane basin in China, situated in the river basin of the middle Yangtze and two of its tributaries, the Min and Chialing rivers. The basin, which has an area of 200,000 sq km, is bordered by the Sino-Tibetan Mountains in the west, the Tsinling and Tapa ranges in the north, and the Kweichow Plateau in the south. It is composed of a layer of Jurassic and Cretaceous red sandstones as much as 4,000 m in thickness (hence the name Red Basin). Shaped like a terraced amphitheater facing south toward the Yangtze River valley, the basin has average elevations of 400–500 m and rises to 1,000 m along the periphery. Flat-topped hills 50–100 m higher than the surrounding land predominate. Artificial terracing of slopes is widespread.

The region has a subtropical monsoonal climate, with a warm winter and wet summer. Annual precipitation ranges from 800 to 1,750 mm, with the maximum in the summer. Because the mountains protect the Szechwan Basin from cold northern winds, the growing season lasts almost the entire year. The rivers are used extensively for irrigation. A lower vegetation belt features broad-leaved forests, which include oak and kastanopsis. Coniferous and fir forests are found above 2,200 m. The cities of Chungking and Chengtu are situated in the Szechwan Basin.