Once a stream has cut its channel closer to base level, downward erosion becomes less dominant. At this point the stream's channel takes on a meandering pattern, and more of the stream's energy is directed from side to side. The result is a widening of the valley as the river cuts away first at one bank and then at the other.The continuous lateral erosion caused by shifting of the stream's meanders produces an increasingly broader flat valley floor covered with alluvium. This feature, called a floodplain is appropriately named because when a river overflows its banks during flood stage, it inundates the floodplain.
Over time the floodplain will widen to the point that the stream is only actively eroding the valley walls in a few places. In fact, in large rivers such as the lower Mississippi River valley the distance from one valley wall to another can exceed 100 miles.