Wei River, or Wei He, a river in China. It is about 450 miles (720 km) long. The Wei's source is in the mountains of southern Gansu province. From there it flows eastward across Shaanxi province through a fertile valley that was one of the earliest centers of Chinese civilization. The historic city of Xi'an is in the valley. The river is navigable for 80 miles (130 km) from the vicinity of Xi'an to where it joins the Huang He, or Yellow River.