The natural border between the U.S. states of Nebraska and Colorado is formed by the South Platte River. This river serves as a key geographical feature, demarcating the boundary between the two states for much of its length. The South Platte River is important for various recreational activities, including fishing, rafting, and hiking along its banks. It also plays a crucial role in agricultural irrigation and water supply for communities within both states, highlighting its significance in regional water management.