Levees built adjacent to river banks prevent point bars from forming. Because many western rivers run through arid landscapes, their riparian forests serve as natural corridors for woodland birds moving about. Young of many birds need these corridors to help them disperse after gaining independence from their parents. Several montane species may follow the ribbons of riparian growth when, after breeding, they descend to lowland rivers to take advantage of the abundant insect prey. Some species greatly extend their ranges along rivers.
Populations can be far from the species' main distribution.
These can serve as springboards for colonizing adjacent rivers or as sources of recruits to supplement smaller and more vulnerable outlying populations. It is thought that this recent expansion will continue, with new recruits being added from their population strongholds in Arizona. Once a tremendous river system with luxuriant and extensive forest, the Sacramento River is now a prime example of the effects of long-term abuse that face nearly all riparian areas in the West. Willow Flycatcher, Warbling and Bell's Vireos have become extirpated from the valley. Among the causes are loss of habitat through farmland conversion, stream channelization, and the building upstream of Shasta Dam.