The longest river in the United States is the Missouri River, which stretches approximately 2,341 miles (3,767 kilometers) from its source in the Rocky Mountains of Montana to its confluence with the Mississippi River in St. Louis, Missouri. It is a vital waterway that has played an essential role in the history and development of the regions it traverses. The river serves as a crucial resource for irrigation, transportation, and recreational opportunities. Its extensive watershed has shaped the geography and ecosystems of the central United States, contributing significantly to the area's agriculture and urban development.