Big Sioux (sō) [key], river, 420 mi (676 km) long, rising in NE S.Dak. and flowing S into the Missouri River. It passes through an agricultural region that produces corn, oats, hogs, and beef cattle. The river's meandering course drops 20 ft (6 m) and services a hydroelectric station there. The Big Sioux forms part of the border between Iowa and South Dakota.