The West has several long rivers that empty into the Pacific Ocean, while the eastern rivers run into the Gulf of Mexico. The Mississippi River forms the easternmost possible boundary for the West today. The Missouri River, a tributary of the Mississippi, flows from its headwaters in the Rocky Mountains eastward across the Great Plains, a vast grassy plateau, before sloping gradually down to the forests and hence to the Mississippi. The Colorado River snakes through the Mountain states, at one point forming the Grand Canyon.