The principal headwaters of the Missouri are the Jefferson, Madison, and Gallatin rivers, which rise high in the Rocky Mts., SW Mont., and join to form the Missouri near Three Forks, Mont. The Missouri's upper course flows north through scenic mountain terrain including Gate of the Mountains, a deep gorge. At Great Falls, Mont., the river enters a 10-mi (16-km) stretch of cataracts that prevented navigation to the upper river and effectively established Fort Benton, Mont., as the head of navigation for 19th-century riverboats. Below Fort Benton the Missouri follows a meandering course east through the unspoiled Missouri Breaks and Fort Peck Lake (behind Fort Peck Dam) then southeast through the dammed Lakes Sakakawea and Oahe and across the Great Plains of the W central United States, crossing Montana, North Dakota, and South Dakota, and forming part of the boundaries of Nebraska, Kansas, and Iowa before crossing Missouri and entering the Mississippi River 17 mi (27 km) N of St. Louis. Nicknamed "Big Muddy" for its heavy load of silt, the brown waters of the Missouri do not readily mix with the gray waters of the Mississippi until c.100 mi (160 km) downstream. The Yellowstone and Platte rivers are the Missouri's chief tributaries.