The name Upper Castor River is for the more northerly, 111 km (69 mi) long section of the original Castor River. Historically, the name Castor River applied to a 230 km (142mi) long stream. However, in the early 20th century, the Little River Drainage District constructed the Castor River Diversion Channel, which bisected the Castor River, causing the northern section of the stream to be diverted into the diversion channel, and separating the southern portion from its original headwaters. In 2007, the Board on Geographic Names (BGN) approved a proposal to rename the two portions Upper Castor River and Lower Castor River.
Historically The Castor River name has been applied to a 230 km (142 mi) long stream since the early 19th century. According to Robert Ramsay’s Missouri Place Name file (1934), Schoolcraft mentioned the Castor River as early as 1818; the name is of French origin and means “beaver.”