The headwaters of the Missouri are in the Rocky Mountains of southwestern Montana, near the continental divide. The river rises in the Jefferson, Madison, and Gallatin rivers, which converge near Three Forks, Montana to form the Missouri. It flows north, through mountainous canyons, emerging from the mountains near Great Falls, where a large cataract historically marked the navigable limit of the river. It flows east across the plains of Montana into North Dakota, then turns southeast, flowing into South Dakota, and along the north and eastern edge of Nebraska, forming part of its border with South Dakota and nearly all its boundary with Iowa, flowing past Sioux City and Omaha. It forms the entire boundary between Nebraska and Missouri, and part of the boundary between Missouri and Kansas. At Kansas City, it turns generally eastward, flowing across Missouri and joins the Mississippi just north of St. Louis. The river is nicknamed "Big Muddy" because of the high silt content in its flow, a feature that is highly visible at its confluence with the Mississippi. The river was of great importance in the westward expansion of the United States. It was acquired by the United States as part of the Louisiana Purchase and explored by the Lewis and Clark Expedition, which successfully used the river to find a route to the Pacific Ocean. During the late 19th century, the river was a primary means of transportation of goods and passengers before the spread of the railroads. The extensive use of paddle steamers on the upper river helped facilitate white settlement of the Dakotas and Montanas, helping spark several of the most intense Indian Wars in the region. In the 20th century, the upper Missouri was extensively dammed for flood control, irrigation, and hydroelectric power. After President Roosevelt signed the Flood Control Act of 1944, the Pick-Sloan Plan turned the Missouri River into the largest reservoir system in North America. There are six dams in three states with four located in South Dakota. The river's course roughly follows the edge of the glaciation during the last ice age. Most of the river's longer tributaries stretch away from this edge, towards the west, draining portions of the eastern Rockies.