Riparian Area Loss and Degradation

Riparian areas are critical ecosystems in the semi-arid landscape of the Colorado Plateau, yet in the last few decades many have been seriously degraded and others entirely lost due to human activities and land use. Overall, a 90% loss of presettlement riparian ecosystems has occurred in Arizona and New Mexico.

The degradation of riparian communities on the Colorado Plateau began in the early 19th century with the near extirpation of the region's beaver population by fur trappers. Beavers play an important role in creating and maintaining riparian areas by cutting trees and building dams. Water retained in beaver ponds during periods of low flow support native fish populations and provide drinking water essential not only for mammal populations, but for some species of birds and bats. The trapping of alluvial sediments by beaver ponds provide opportunities for new plant growth.

A dam on the Virgin River downstream of Zion National Park. Photo © Ray Wheeler.

By contrast, human diversion or impoundment of free-flowing water by dams, diversions, irrigation, or channelization has been a major factor in the degradation of the natural functions of riparian areas. Without natural hydrologic systems, water tables have lowered, and surface sediments have dried out. Cottonwoods are particulary susceptible to water stress and may decline as groundwater becomes less available.

With less flooding, there is less channel shifting and less suitable habitat for establishment of cottonwood and willow seedlings, which are dependent on recently inundated sediments to become established. Diverse mosaics of riparian vegetation created by shifting of river channels have decreased or been eliminated in many riparian systems on the Colorado Plateau. Where existing riparian forests have aged without replacement, they have become a monoculture of maturing trees that eventually senesce, die, or become victims of fires.

Overgrazing by domestic livestock has been a major factor in the alteration and degradation of riparian areas. Heavy grazing, whether by big game or livestock, deteriorates stabilizing vegetation, erodes banks, and causes declines in water storage capacity and quality. In some cases gullying or arroyo cutting occur. In others, stream beds become wider and streambeds shallower, water temperatures rise, and fish and aquatic invertebrate habitat quality declines.

Riparian systems at lower elevations on the Colorado Plateau are now increasingly characterized by a reduction of plant species diversity and density. Overgrazing of palatable native species such as willows and cottonwood saplings, combined with the introduction of less palatable nonindigenous species such as Russian olive and tamarisk, has also contributed to changes in overall plant community structure. Tamarisk, introduced to the Colorado Plateau in the 1950s, has been particularly devastating, outcompeting cottonwood and willow, and dominating lower elevation riparian systems throughout the region. Its establishment introduces a regime of episodic fire, which researchers believe is uncommon in most native riparian woodlands.

Road building, logging, construction and other development has caused additional degradation of riparian areas, especially through bank erosion. Additional nutrients and fertilizers added to stream systems by agricultural runoff and sewage treatment facilities have resulted in reductions in water quality and increased eutrophication.

As natural riparian zones are being lost, so are their associated faunas. The highest percentages of threatened fish in the United States are found in the Colorado Plateau states and California. Although only 36 native freshwater fish species formerly lived in the Colorado River basin, the largest watershed in the Southwest, species-level endemism is high at 64%. The number of nonindigenous fishes introduced into the Colorado River basin is 72, twice the number of native fishes. Four of the five fishes that evolved in large rivers in the Colorado River basin are listed as endangered, and the fifth is listed as a sensitive species. The few remaining fish stocks native to smaller riparian systems on the Colorado Plateau survive only in those areas with intact riparian habitats, often in remote upper-elevation watersheds.

The responses of riparian bird communities to changes due to grazing have been particularly well studied. At some sites 40% of riparian bird species were negatively affected by livestock grazing, and a negative correlation between recent cattle grazing and abundance of several riparian birds was found.

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