Red River of the South  The Red River, or sometimes the Red River of the South, is a major river in the Southern United States. It 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. 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. The Red River is the second-largest river basin in the southern Great Plains. It rises in two branches in the Texas Panhandle and flows east, where it acts as the border between the states of Texas and Oklahoma. It forms a short border between Texas and Arkansas before entering Arkansas, turning south near Fulton, Arkansas, and flowing into Louisiana, where it flows into the Atchafalaya River. The total length of the river is , with a mean flow of over at the mouth. The North Fork Red River meets the southern and largest fork near the Texas–Oklahoma border. The southern fork, which is about long, is generally called the Prairie Dog Town Fork. It is formed in Randall County, Texas, near the county seat of Canyon, by the confluence of Tierra Blanca Creek and intermittent Palo Duro Creek (Not to be confused with another Palo Duro Creek 75 miles to the north which drains into the North Canadian River). The Red River turns and flows southeast through Palo Duro Canyon in Palo Duro Canyon State Park at an elevation of , then past Newlin, Texas, to meet the Oklahoma state line. Past that point, it is generally considered the main stem of the Red River. Near Elmer, Oklahoma, the North Fork finally joins, and the river proceeds to follow a winding course east through one of the most arid parts of the Great Plains, receiving the Wichita River as it passes the city of Wichita Falls. Near Denison, the river exits the eastern end of Lake Texoma, a reservoir formed by the Denison Dam. The lake is also fed by the Washita River from the north. After the river flows out of the southeastern end of the lake, it runs generally east towards Arkansas and receives Muddy Boggy Creek before turning southward near Texarkana.