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.
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.
Almand, J. and Krohn, W. 1979. The position of the Bureau of Land Management on the protection and management of riparian ecosystems. Pp. 259-361 In: Johnson, R. and McCormick, F., editors. Strategies for protection and management of floodplain wetlands and other riparian ecosystems: Proceedings of the symposium, 1113 December 1978. General Technical Report WO-12. U.S. Forest Service, Washington, D.C.
Ames, C. R. 1977. Wildlife conflicts in riparian management: Grazing. Pp. 49-58 In: Johnson, R. R. and Jones, D. A., editors. The importance, preservation and management of the riparian habitat. General Technical Report RM-43. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO.
Anderson, B. W. and Ohmart, R. D. 1979. Riparian revegetation: An approach to mitigating for a disappearing habitat in the Southwest. Pp. 481-487 In: Swanson, G. A., editor. The mitigation symposium: A national workshop on mitigating losses of fish and wildlife habitats. General Technical Report RM-65. USFS, Rocky Mountain Forest and Range Experiment Station, Fort Collins.
Armour, C., Duff, D. and Elmore, W. 1991. The effects of livestock grazing on riparian and stream ecosystems. Fisheries 16: 7-11.
Barth, R. C. and McCullough, E. J. 1988. Livestock grazing impacts on riparian areas within Capitol Reef National Park. Capitol Reef National Park, Torrey, UT.
Benenati, P. L., Shannon, J. P. and Blinn, D. W. 1998. Desiccation and recolonization of phytobenthos in a regulated desert river: Colorado River at Lees Ferry, Arizona, USA. Regulated Rivers 14: 519.
Brock, J. H. 1994. Tamarix spp. (salt cedar), an invasive exotic woody plant in arid and semi-arid riparian habitats of western USA. Pp. 27-44 In: de Waal, L. C., Child, L. E., Wade, P. M. and Brock, J. H., editors. Ecology and management of invasive riverside plants. John Wiley and Sons Ltd, Chichester, NY.
Brookshire, D. S., McKee, M. and Schmidt, C. 1996. Endangered species in riparian systems of the American west. Pp. 238-241 In: Shaw, D. W. and Finch, D. M., editors. Desired future conditions for southwestern riparian ecosystems: bringing interests and concerns together. General Technical Report RM-272. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO.
Carothers, S. W. and Dolan, R. 1982. Dam changes on the Colorado River. Natural History 91: 74-84.
Carothers, S. W. and Brown, B. T. 1991. The Colorado River through Grand Canyon: Natural history and human change. University of Arizona Press, Tucson, 235 pp.
Christensen, E. M. 1962. The rate of naturalization of Tamarix in Utah. American Midland Naturalist 68: 51-57.
Cooper, D. J., Merritt, D. M., Anderson, D. C. and Chimner, R. A. 1999. Factors controlling the establishment of Fremont cottonwood seedlings on the Upper Green River, USA. Regulated Rivers: Research and Management 15: 419-440.
Deacon, J. E. 1988. The endangered woundfin and water management in the Virgin River, Utah, Arizona, Nevada. Fisheries 13: 18-24.
DeBano, L. F. and Schmidt, L. J. 1989. Improving Southwestern riparian areas through watershed management. Report RM-182. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO, 33 pp.
Denevan, W. M. 1967. Livestock numbers in nineteenth-century New Mexico and the problem of gullying in the Southwest. Annals of the Association of American Geography 57: 691-703.
Detenbeck, N. E., DeVore, P. W., Niemi, G. J. and Lima, A. 1992. Recovery of temperate-stream fish communities from disturbance: A review of case studies and synthesis of theory. Environmental Management 16: 33-53.
Douglas, E. 1954. Phreatophytes: Water hogs of the west. Land Improvement 1: 8-12.
Duce, J. T. 1918. The effect of cattle on the erosion of canyon bottoms. Science 47: 450-452.
Fleischner, T. L. 1994. Ecological costs of livestock grazing in western North America. Conservation Biology 8: 629-644.
General Accounting Office, 1988. Public rangelands: some riparian areas restored but widespread improvement will be slow. General Accounting Office, Washington, D.C., 85 pp.
Gilles, C., Bravo, L. and Watahomigie, D. 1991. Uranium mining at the Grand Canyon: What costs to water, air, and indigenous people? The Workbook 16: 2-17.
Graf, W. L. 1986. Fluvial erosion and federal public policy in the Navajo Nation. Physical Geography 7: 97-115.
Harris, J. H. and Silveira, R. 1999. Large-scale assessments of river health using an index of biotic integrity with low diversity fish communities. Freshwater Biology 41: 235-252.
Hunter, W. C., Ohmart, R. D. and Anderson, B. W. 1988. Use of exotic saltcedar (Tamarix chinensis) by birds in arid riparian systems. The Condor 90: 113-123.
Johnson, J. E. 1987. Reintroducing the natives: Colorado squawfish and woundfin. Pp. 118-124 In: Proceedings of the Desert Fishes Council. XVI-XVIII. Desert Fishes Council, Bishop, CA.
Johnson, R. R. 1991. Historic changes in vegetation along the Colorado River in the Grand Canyon. Pp. 178-206 In: Marzolf, G. R., editor. Colorado River ecology and dam management. National Academy Press, Washington, D.C.
Karp, C. A. and Tyus, H. M. 1990. Humpback chub (Gila cypha) in the Yampa and Green Rivers, Dinosaur National Monument, with observations on roundtail chub (G. robusta) and other sympatric fishes. Great Basin Naturalist 50: 257-264.
Kauffman, J. B., Beschta, R. L., Otting, N. and Lytjen, D. 1997. An ecological perspective of riparian and stream restoration in the western United States. Fisheries 22: 12-24.
Kay, C. E. 1994. The impact of native ungulates and beaver on riparian communities in the Intermountain West. Natural Resources and Environmental Issues 1: 23-44.
Knopf, F. L. 1989. Riparian wildlife habitats: more, worth less, and under invasion. Pp. 20-22 In: Mutz, K., Cooper, D., Scott, M. and Miller, L., editors. Restoration, creation, and management of wetland and riparian ecosystems in the American West. Society of Wetland Scientists, Rocky Mountain Chapter, Boulder, CO.
Krueper, D. J. 1993. Effects of land use practices on western riparian ecosystems. Pp. 321-330 In: Finch, D. M. and Stangel, P. W., editors. Status and management of Neotropical migratory birds. General Technical Report RM-229. U.S. Forest Service.
Leiner, S. 1996. The habitat quality index applied to New Mexico streams. Hydrobiologia 319: 237.
Modde, T., Scholz, A. T., Williamson, J. H., Haines, G. B., Burdick, B. D. and Pfeifer, F. K. 1995. An augmentation plan for razorback sucker in the Upper Colorado River Basin. American Fisheries Society Symposium 15: 102-111.
Neary, D. G. and Medina, A. L. 1995. Geomorphic responses of a montane riparian habitat to interactions of ungulates, vegetation, and hydrology. In: Shaw, D. W. and Finch, D. M., editors. Desired future conditions for Southwestern riparian ecosystems: Bringing interests and concerns together. General Technical Report RM272. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO.
Ohmart, R. D. 1994. The effects of human-induced changes on the avifauna of western riparian habitats. Studies in Avian Biology 15: 273-285.
Rieger, J. 1992. Western riparian and wetland ecosystems. Restoration & Management Notes 10: 52-55.
Rinne, J. N. 1996. Desired future condition: Fish habitat in southwestern riparian-stream habitats. Pp. 336-345 In: Shaw, D. W. and Finch, E. M., editors. Desired future conditions for southwestern riparian ecosystems: Bringing interests and concerns together. General Technical Report RM-272. U.S. Forest Service Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO.
Rinne, J. N. 1999. Fish and grazing relationships: The facts and some pleas. Fisheries 24: 12-21.
Schultz, T. T. and Leininger, W. C. 1990. Differences in riparian vegetation structure between grazed areas and exclosures. Journal of Range Management 43: 295-299.
Schultz, T. T. and Leininger, W. C. 1991. Nongame wildlife communities in grazed and ungrazed riparian sites. Great Basin Naturalist 51: 286-292.
Sogge, M. K. 1995. Southwestern willow flycatcher surveys along the San Juan River, 1994-1995: Final report to the Bureau of Land Management, San Juan Resource Area. National Biological Service, Colorado Plateau Research Station, Northern Arizona University, Flagstaff, AZ, 27 pp.
Sogge, M. K., Marshall, R. M., Sferra, S. J. and Tibbitts, T. J. 1997. A southwestern willow flycatcher natural history summary and survey protocol. Technical Report NPS/NAUCPRS/NRTR-97/12. National Park Service, Washington, D.C.
Sogge, M. K., Tibbitts, T. J. and Petterson, J. R. 1997. Status and breeding ecology of the southwestern willow flycatcher in the Grand Canyon. Western Birds 28: 142.
Stevens, L. E. and Waring, G. L. 1988. Effects of post-dam flooding on riparian substrates, vegetation, and invertebrate populations in the Colorado River corridor in Grand Canyon. Glen Canyon Environmental Studies Report No. 19. Bureau of Reclamation, Flagstaff, AZ.
Stevens, L. E., Schmidt, J. C., Ayers, T. J. and Brown, B. T. 1995. Flow regulation, geomorphology, and Colorado River marsh development in the Grand Canyon, Arizona. Ecological Applications 5: 1025-1039.
Stromberg, J. C. and Chew, M. K. 1997. Herbaceous exotics in Arizona's riparian ecosystems. Desert Plants 13: 11.
Swenson, E. A. and Mullins, C. L. 1985. Revegetating riparian trees in southwestern floodplains. Pp. 135-138 In: Johnson, R. R., Ziebell, C. D., Patton, D. R., Ffolliott, P. F. and Hamre, R. H., editors. Riparian ecosystems and their management: reconciling conflicting uses. General Technical Report RM-120. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO.
Taylor, D. M. 1986. Effects of cattle grazing on passerine birds nesting in riparian habitat. Journal of Range Management 39: 254-258.
Tremble, M. 1993. The Little Colorado River. Pp. 283-289 In: Tellman, B., Cortner, H. J., Wallace, M. G., DeBano, L. F. and Hamre, R. H., editors. Riparian management: common threads and shared interests. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO.