Abstract About 2000 years ago, the Mississippi River avulsed westwards and captured the Red River of the South, uprooting vegetation and causing bank instability, resulting in a series of log jams known as the Great Red River Raft. The Great Raft presented a major navigation challenge during the settlement of the Red River Valley from the seventeenth century until its removal in the 1890s. The Great Raft extended c . 100–300 km up-river from Natchitoches, Louisiana to the Louisiana–Arkansas boundary. In the nineteenth century, it contributed to a major military failure by the Union Army and Navy during the 1864 Red River Campaign of the American Civil War. While planning the Red River Campaign, Union commanders made tactical errors by overlooking unique geological aspects of the Red River, including its flashy nature, high sediment load, anastomosing channels and rapids. The Confederates used their knowledge of the Great Raft and the Red River’s morphology to attack the Union’s river fleet while in a position of restricted manoeuverability. The Confederates used the topographically higher defensive positions of natural levee deposits along the Red River to keep the Union’s land and naval forces from supporting each other during the Campaign.