Although rain is infrequent in the Sonoran Desert, when it does fall, it comes down fast and in large quantities. Runoff from summer rain storms quickly runs into seasonal channels called washes. Desert trees take advantage of the bonus water along these washes, and grow significantly taller than the same species growing just 100 feet from a channel's edge.
Trees found along desert washes include the velvet mesquite (Prosopis velutina), blue palo verde (Parkinsonia florida), desert willow (Chilopsis linearis), smoke tree (Dalea spinosa) and catclaw acacia (Acacia greggii).
Although water levels have declined during the past 100 years, Arizona once had many rivers running year-round. With population growth and groundwater pumping, these rivers are slowly disappearing. Some water still flows in a few rivers, but often it disappears underground.
Tree species along these rivers are considered riparian species because they depend on regular water provided by rivers. The Arizona cottonwood (Populus fremontii), velvet ash (Fraxinus velutina) and netleaf hackberry are among native tree species found along sections of riparian habitat. Also found along rivers is the non-native and invasive tamarisk (Tamarix species), which is endangering riparian habitats by crowding out native trees and choking river channels.
The Sonoran Desert is divided into different habitats that are based on elevation and annual rainfall amounts. The Sonoran desertscrub grows in two habitats, the Arizona Upland subdivision and the Lower Colorado River Valley subdivision.
Away from washes and rivers, some of the same species found along washes grow, but they are smaller in stature. In addition to velvet mesquite and catclaw acacia, other tree species found along washes in this habitat include the desert ironwood (Olneya tesota), the foothills palo verde (Parkinsonia microphyllum) and the whitethorn acacia (Acacia constricta). Occasionally, the blue palo verde is found along minor drainages.
This subdivision is the most arid part of the Sonoran Desert and so has fewer trees than the upland division. Along drainages, however, velvet mesquite, blue palo verde, ironwood and smoketree can sometimes be found.
