The Missouri flows free for 59 miles below Gavins Point Dam and for 39 miles below Fort Randall Dam, where sections of the national recreational river offer glimpses of the once undammed, unchannelized river that played a major role in America's settlement and commerce. Because the Missouri loses elevation about one foot for every mile, its waters do not pick up much speed, so instead of cutting straight down through the valley, the river winds around high spots, solid rock, and resistant soils. Behind the dams, the waters slow even more and dump their loads of sand and dirt, while below the dams, relatively clear waters erode sandy riverbanks and pick up new sediment. The river shapes the landscape in many ways: meanders form as riverbanks bounce the river's energy back and forth, with faster water eroding the outside of a bend and slower water depositing sediment on the inside; wetlands may form during major floods when the river cuts a new channel through a meander and diverts most of the water; snags, or dead trees in the river, collect sediment on their downstream side; oxbow lakes may form where a high water table fills cut-off meanders left dry by the main channel; islands develop when the river divides and follows both an older, deeper channel and a newer, shallower one around a piece of land; and sandbars form when fast-moving water loses energy downstream and drops soil taken from the river bottom and banks.