Complex sentence: In the United States, the Red River is sometimes called the Red River of the North, to distinguish it from the Red River that is a tributary of the Atchafalaya River, and that forms part of the border between Texas and Oklahoma.
Split into simpler sentences: In the United States, the Red River is sometimes called the Red River of the North. This distinguishes it from the so-called Red River of the South, a tributary of the Atchafalaya River that forms part of the border between Texas and Oklahoma.