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Riparian environments are river bank environments, although in desert regions the term tends to be used more broadly to include temporary water courses such as gullies and washes that receive run-off water from the mountains during the wet season. Even when these washes are dry, they tend to retain a higher moisture content in the soil. The result is an unusually lush vegetation compared with that on the surrounding land.

Transition from a bajada with saguaro cacti (at the base of the hill) down to the lusher, green shrub vegetation on the bank of a dry wash (at the bottom of this picture). Sonoran Desert, Arizona.

A small, spring-fed pool in the Sonoran Desert (Organ Pipe National Monument). This "oasis" is one of the few desert environments where the desert pupfish (below) occurs naturally.

Male desert pupfish (about 8 cm long). The female does not have the blue coloration.

River courses (even if they contain no free-flowing water for most of the year) are often conspicuous by the growth of cottonwood trees (poplars) in desert environments of southwestern USA.

Cottonwoods along a river bank. The flat-topped hills are mesas, formed naturally by erosion of uplifted areas of sedimentary rocks. This erosion is slowed by any bands of harder rocks, clearly seen at the left of the image where the mesa is capped by a band of harder rock.


In desert regions of southwestern USA the rivers and other water courses can provide ideal habitats for invading organisms. The image below shows part of the Colorado River at Yuma (extreme south of Arizona) where the river fllod plain has been dominated by thick growth of tamarisk trees (Tamarix ramosissima and related species) which were introduced into the USA from Europe or Asia, as fast-growing trees to serve as windbreaks along fields.

Almost all the vegetation seen in this image is a thick growth of tamarisk, an exotic "alien" plant that thrives in wet habitats of southwestern USA.

Close-up of part of the dense thicket of tamarisk

Tamarisk flower spikes

Tamarisk leaves are small and scale-like, clasping the stems. The arrangement of scale leaves resembles that of cedar trees - hence the common name "saltcedar" for tamarisk.

Tamarisk is so successful as an invader because it produces prolific seeds which germinate readily. It displaces the native riparian trees such as mesquite, willow and cottonwood because it is much more salt-tolerant than them. It achieves this by excreting excess salt onto the leaf surfaces. Quite apart from its domination of the water courses, tamarisk is detrimental because it transpires a large amount of water - more so that desert plants that evolved to be adapted for water conservation.


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