Sichuan Basin

The Sichuan Basin (Chinese: 四川盆地; pinyin: Sìchuān Péndì), formerly transliterated as the Szechwan Basin, sometimes called the Red Basin, is a lowland region in southwestern China. It is surrounded by mountains on all sides and is drained by the upper Yangtze River and its tributaries. The basin is anchored by Chengdu, the capital of Sichuan province, in the west, and the direct-administered municipality of Chongqing in the east. Due to its relative flatness and fertile soils, it is able to support a population of more than 100 million. In addition to being a dominant geographical feature of the region, the Sichuan Basin also constitutes a cultural sphere that is distinguished by its own unique customs, cuisine and dialects. It is famous for its rice cultivation and is often considered the breadbasket of China. In the 21st century its industrial base is expanding with growth in the high-tech, aerospace, and petroleum industries.

The Sichuan Basin is an expansive 229,500 km² (88,600 sq mi) lowland region in China that is surrounded by upland regions and mountains. Much of the basin is covered in hilly terrain. The basin covers the eastern third of Sichuan Province and the western half of Chongqing Municipality.^{[citation needed]}

The westernmost section of the Sichuan Basin is the Chengdu Plain, occupied by Chengdu, provincial capital of Sichuan. The Chengdu Plain is largely alluvial, formed by the Min River and other rivers fanning out when entering the basin from the northwest. This flat region is separated from the rest of the basin by the Longquan Mountains. The central portions of the Sichuan Basin are generally rolling, covered by low hills, eroded remnants of the uplifted Sichuan Basin floor. In some parts of the extreme northern Basin and in Weiyuan County in the southwest, there are ancient dome-shaped low mountains in their own right. The Jialing River enters from the north and flows across the entire width of the Sichuan Basin to meet the Yangtze at Chongqing. Northeast of Chongqing, the Yangtze cuts an outlet through the mountains at the eastern edge of the basin known as the Three Gorges. Other significant rivers almost wholly within the Sichuan Basin include the Tuo River, the Fu River, and the Qu River.

Due to the surrounding mountains, the Sichuan Basin often experiences fog and smog as a result of temperature inversion caused by the basin's convective layer being capped by a layer of air moving east across the Tibet Plateau.

A moist, often overcast, four-season climate dominates the basin, with cool to mild winters occasionally experiencing frost, and hot, very humid summers. The intensity of summer varies rather widely throughout the basin, depending on location. Generally, the climate is warmer and wetter in the eastern parts of the Sichuan Basin. The basin's climate is classified as humid subtropical under Koppen classification.^{[citation needed]} The entirety of the Sichuan Basin is drained by the Yangtze River and its tributaries. The main stem of the Yangtze, the Jinsha River, enters the basin in the south at Yibin where it meets the Min River, which enters the basin from the northwest at Dujiangyan City and flows southerly to meet the Jinsha at Yibin where together they form the Yangtze in name. The Dadu River enters from the west and joins the Min at Leshan.

The Sichuan Basin forms the rigid northwest edge of the Yangtze tectonic plate. The Yangtze Plate's complex relationship with the surrounding Eurasian Plate is evidenced at its margins. Orogeny formed by the Indian Plate's collision with Eurasia has compressed against the Sichuan Basin's western edge, most notably along the Longmenshan Fault, the epicenter of the 2008 Sichuan Earthquake. The basin's rigidity withstands much of the Tibetan Plateau's eastern movement, but dramatic folds have formed within the Yangtze Plate along the Sichuan Basin's eastern edges. Here, ancient faults interact with the Daba Mountains, themselves a result of pressure between the Yangtze and Eurasian Plates in a perpendicular direction.

Until 6 million years ago, a large lake filled the Sichuan Basin. The basin's soils today are largely exposed red sandstone, leading to the "Red Basin" nickname for the region. The Sichuan Basin's well preserved Jurassic layers have proven valuable to paleontology, such of those of the Shaximiao Formation, near Zigong, which preserves abundant remains of dinosaurs and other prehistoric animals.

Originally, the Sichuan Basin was covered by the Sichuan Basin evergreen broadleaf forests. With human settlement, agriculture has taken root across most of the fertile basin and reduced the original forest to small patches on hills and mountains including Mount Emei. The extensive ridges in the eastern Sichuan Basin preserve elements of the original forests. A greater variety of natural landscapes and wildlife have been at least partially preserved in the mountains surrounding the basin where human settlement has been less intensive. The natural ecosystems of these mountains have been classified by the World Wide Fund for Nature as the Qionglai-Minshan conifer forests to northwest and the Daba Mountains evergreen forests to the northeast and east.