A stream, however, needs more than water to form a healthy riparian area. A stream in its natural state tends to meander and over time changes its course over a wide flood plain. Cottonwoods may germinate along a new channel, while mature cottonwoods still grow along an old channel.
Terraces tend to form and different vegetation is found on successively higher terraces.
These conditions often are not possible in a populated area such as downtown Tucson, where buildings exist along the banks, and flood control structures constrain the river within a narrow channel. Where the stream channel is in a relatively natural state, as is the case with many washes, restoration does not pose great difficulties. The addition of water may be all that is needed. The Rillito Creek at Craycroft Road, for example, had a lush cottonwood and walnut forest until very recent times. Pumping has depleted the aquifer to the point that the trees are stressed.