Wei (wā) [key], river, c.450 mi (720 km) long, rising in SE Gansu prov. and flowing E through Gansu and Shaanxi provs. to the Huang He. Its wide, alluvial valley was the site of some of the earliest centers of Chinese civilization (such as the Chou dynasty, 1027–256 B.C.) and is now the agricultural and population center of Shaanxi prov. Cotton, wheat, millet, and fruits are extensively grown.