Riparian habitats, at the interface between wet and dry systems, are defined by the plants that inhabit them. Riparian plants depend on an intact hydrological regime where groundwater is maintained and natural surface flows occur. On the Colorado Plateau, many hydrological regimes are characterized by annual cycles of flooding and minimal flows, which determine sediment availability.
Several regionally significant rivers, including the San Juan, Escalante, Sevier, Little Colorado, Green, and Colorado, cut through the stark mesas and broad plains of the Colorado Plateau. Continuous corridors of riparian vegetation once covered hundreds of miles along desert and mountain rivers. Besides forested riparian communities, there were riparian shrublands, marshlands, and grasslands. These plant communities were found at elevations from high wet meadows and cienegas, to tree-banked streams, to slack water sloughs and marshesthe alpine, montane, and floodplains-plains riparian ecosystems.
Along many rivers and creeks on the Colorado Plateau at middle elevations, from about 5000 feet to nearly 8,000 feet, several broadleaf trees commonly form riparian gallery forests or woodlands. These communities are especially well-developed where environmental conditions permit a mix of some lower-elevation species with some higher-elevation species. Common native trees and shrubs, depending on location and elevation, include narrowleaf cottonwood (Populus angustifolia), box-elder (Acer negundo), bigtooth maple (Acer grandidentatum), Rocky Mountain maple (Acer glabrum), water birch (Betula occidentalis), aspen (Populus tremuloides), thin-leaf alder (Alnus tenuifolia), New Mexico locust (Robinia neomexicana), Scouler willow (Salix scouleriana), and arroyo willows (Salix lasiolepis).
At higher elevations a large proportion of the Colorado Plateau's streams pass through the upland montane forests of mixed-conifer and ponderosa pine communities. The riparian zones themselves are usually narrow, often following relatively steep stream channels in restricted valleys. White fir and blue spruce are the common conifers in and adjacent to these montane riparian ecosystems. The streams usually flood from snowmelt in the spring, and many riparian species depend on over-bank flooding for seed transport and burial in fresh, fertile alluvial sediments. Seed shedding and flooding tend to coincide.
Although the riparian habitats of these rivers and their tributaries represent less than 1 percent of the total acreage of public lands in the 11 western states, about 72% of all reptiles, 77% of all amphibian species, 80% of all mammals, and 90% of all bird species which occur regularly in the Colorado Plateau region routinely use riparian areas for food, water, cover or migration routes. About 30% of the region's bird species use wetlands and other aquatic areas to the exclusion of upland habitats. Wetland and riparian habitats also support a disproportionate number of species that are of concern because they migrate to neotropical areas, have small continental populations, or are declining.
Since settlement by Europeans, riparian forests of all types have suffered enormous declines due to destruction, conversion to other uses, or significant degradation in structure, function, or composition. Non-native saltcedar or tamarisk (Tamarix ramosissima) is now a dominant riparian shrubby tree in the Colorado River basin below 6,000 feet, spread rapidly throughout the system via wind-dispersed seeds. Overall, a 90% loss of presettlement riparian ecosystems has occurred in Arizona and New Mexico. For an essay on the causes and consequences of riparian loss and degradation, click here.
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