A stream valley consists of a channel and the surrounding terrain that contributes water to the stream. Thus it includes the valley floor, which is the lower, flatter area that is partially or totally occupied by the stream channel, and the sloping valley walls that rise above the valley floor on both sides. Most stream valleys are much broader at the top than is the width of their channel at the bottom. This would not be the casc if the only agent responsible for eroding valleys were the streams flowing through them. The sides of most valleys are shaped by a combination of weathering, overland flow, and mass wasting. In some arid regions, where weathering is slow and where rock is particularly resistant, narrow valleys having nearly vertical walls are common. Stream valleys exist in a continuum from narrow, steep-sided valleys to those that are so flat and wide that the valley walls are not discernable.