<Tr> <Th_colspan="2"> Basin features </Th> </Tr> <Tr> <Th> Basin size </Th> <Td> 65,595 sq mi (169,890 km) </Td> </Tr> <P> The Red River, or sometimes the Red River of the South, is a major river in the southern United States of America . The river was named for the red - bed country of its watershed . It is one of several rivers with that name . Although it was once a tributary of the Mississippi River, the Red River is now a tributary of the Atchafalaya River, a distributary of the Mississippi that flows separately into the Gulf of Mexico . It is connected to the Mississippi River by the Old River Control Structure . </P> <P> The south bank of the Red River formed part of the US - Mexico border from the Adams - Onís Treaty (in force 1821) until the Texas Annexation and the Treaty of Guadalupe Hidalgo . </P>

How did the red river get its name
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