Three riparian ecosystems, delineated by elevation, are recognized in the arid and semi-arid southwestern United States and northern Mexico. Riparian vegetation that occurs along the flood plain of stream channels are typically composed of herbaceous species of Carex, Eleocharis, Juncus, and Scirpus and produce the characteristic dark green edge along the channel systems. Woody plants, including saltcedar (Tamarix pentandra), sycamore (Platanus wrightii), and cottonwood (Populus fremontii), that are often associated with riparian ecosystems are typically found higher up on the terraces next to the flood plains.
Elevation Makes a Difference
Above 7,000 ft, herbaceous species of sedges (Carex), rushes (Juncus), spikerushes (Eleocharis), and bulrushes (Scirpus) predominate along the edge of the stream channels. Willow, chokecherry (Prunus virens), boxelder (Acer negundo), Rocky Mountain maple (A. glabrum), and various coniferous tree species occupy the higher terraces.
Riparian ecosystems between 3,500 and 7,000 ft contain the greatest number of plant species and the greatest canopy cover. Besides the characteristic herbaceous plants along the flood plain, cottonwood, willow (Salix spp.), sycamore, ash (Fraxinus velutina), and walnut (Juglans major) are typically found on the terraces, with 3 or 4 species often occurring together.
In ecosystems below 3,500 ft, many of the ephemeral streams have broad alluvial floodplains that can support herbaceous plants and terraced bottoms that often support high densities of deep-rooted trees including saltcedar, sycamore, cottonwood, palo verdes (Cercidium spp.), and other species.
Water Scarcity is Relative
Because of the abundance of water, plants, and animals, riparian areas provide valuable recreation opportunities as well as forage for livestock and wildlife in an otherwise arid environment. Riparian ecosystems are prime habitats for many game and non-game species of wildlife and fish.
Collectively, climatic characteristics of riparian ecosystems exhibit a wide range of conditions due to large elevational differences and distributions of associated mountain ranges and highlands. The key characteristic of the riparian system is the availability of water throughout the year or at least during the growing season.
Soils at the higher elevations generally consist of consolidated or unconsolidated alluvial sediments from parent materials of the surrounding uplands. Soil depths vary in riparian ecosystems, depending upon the stream gradient, topographic setting, and parent materials. Soils on the flood plains at lower elevations consist of recent depositions, tend to be uniform within horizontal strata, and exhibit little development. The alluvial soils in all ecosystems are subject to frequent flooding and, as a consequence, are characterized by a range of textures. Riparian ecosystems vary from narrow, deep, steep-walled canyon bottoms, to intermediately exposed sites with at least one terrace or bench, to exposed, wide valleys with meandering streams.
There are currently about 500 images available in the image database illustrating various aspects and conditions found in riparian ecosystems in the southwestern United States. These can be accessed after geting into the database by using the key word riparian.