The second largest adjustment to a change in discharge is in velocity, and the smallest is in width. As one proceeds downstream in a river network, however, the increase in discharge almost always accompanying the increase in drainage area is accommodated mostly by an increase in width, with a smaller rate of increase in depth. Thus rivers become relatively as well as absolutely wider as one proceeds downstream. Average velocity also usually increases downstream in a river system, but at a slow rate. Exceptions to the downstream increase in discharge that characterizes most stream networks occur in arid regions.
Channels there remain dry for most of the year.
As the water proceeds along the previously dry channel, it gradually infiltrates, causing discharge to decrease downstream. The slope of a river channel decreases between its source and its mouth, and the longitudinal profile of a well-established stream is thus a smooth curve, concave upward. During a flood, for example, a river has a high velocity and is capable of carrying more sediment than normal. It therefore cuts down into the river bottom, picking up more sediment and decreasing its slope until its velocity matches its sediment load. When the flood ends and the discharge returns to normal, the river rids itself of the excess sediment by redepositing it in the channel, building up its slope until velocity and sediment load are once more in balance.