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as 'one possible solution; however, "thip wo,uld require such a major c:ommitment of resources that it seems impractical at this time. Therefore, we must face the reali- zation that we have created a new environment in the Grand Canyon and that the physical and ecologi- cal adjustments presently under- way are inevitable. The system is now so modified that the earlier natural state can no longer serve as the standard. For this reason our focus should be on the processes and rates of adjust- ment to the new river regime. To do this, new data and new investi- gations are needed. In the mean- time, however, the National Park Service must face several difficult decisions concerning regulation of man's use of the river. If human impact on the physical environ- ment is high compared to the post- dam degradation by fluvial and eo- lian processes, then low quotas are a certainty. If, on the other hand, the human impact is low compared to' the post-dam erosion rates, then use may be restricted more by so- cial concerns-for the quality of the wilderness experience, for example -than by environmental concerns. Since 1970 the National Park Ser- vice has, been limiting use of the river to approximately 10,000 visi- tors a year; this level is probably the upper limit for the admittedly subjective criterion of a true wilder- ness experience. Summary The Colorado River in its natural state often discharged more than 80,000 cfs and carried an average of 0.38 million tons of sand and silt a day through the Grand Canyon. This high water scoured the bottom of the canyon and eroded and rede- posited material on the river shores, keeping the river channel navigable for small boats and rafts. In 1963, the river was impounded by Glen Canyon Dam, and the water released by the dam is a tur- bid-free, powerful erosive agent. Daily the discharge varies from 4,000 cfs to about 20,000 cfs, creat- ing many effects and problems. The sediment-free water erodes the existing beach areas but does not resupply the sand lost either to the wind or directly into the river. The reduced discharge is le.ss capable of clearing the river channel of flash- flood material brought in by tribu- taries, thus creating hazardous rap- ids and falls that may eventually become unnavigable. Diurnal varia- tion in the river's discharge has led to ecological changes along the can- yon bottom: exotic plants, for ex- ample, are rapidly displacing indig- enous species. Increased use of the ~river by man in recent years ap- pears to be contributing to the ero- sion problem; however, this impact has not been quantified. At this time the future of the river bars and terraces is unclear. The system is rapidly approaching a new state at the present rate of change; and it is clear that Emory C. Kolb, one of the pioneer river- men on the Colorado, was correct when he said, "No one will ever know the Colorado as it really was. It's too late" (10). References 1. Robert Wallace. 1972. The Grand Can- yon: The American Wilderness. New York: Time-Life Books, p. 23. 2. Mark A. Boster. 1972. Co Iura do River Trips within the Grand Canyon Nation- al Park and Monument: A Socioeco- nomic Analysis. Technical Report on Hydrology and Water Resources, Rept. ::10. Tucson: Univ. of Arizona, p. 24. 3. Kenneth W. Hamblin and J. Keith Rigby. 1969. Guidebook to the Colorado River, part 2. Brigham Young Univ. Geologic Studies, vol. 16. Provo, Utah: Brigham Young Univ., p. 24. 4: Luna B. Leopold. 1969. The rapids and the pools: Grand Canyon. In The Colo- rado River Region and John Wesley Powell. Geological Survey Professional Paper 669-D. Washington: U.S. Gov't. Printing Office, p. 135. 5. W. O. Smith, C. P. Vetter, and G. B. Cummings. 1960. Comprehensive Sur- vey of Sedimentation in Lake Mead, 1948-1949. Geological Survey Profes- sional Paper 295. Washington: U.S. Gov't. Printing Office, p. 196. 6. David R. Harris. 1971. Recent plant invasions in the arid and semi-arid Southwest of the United States. In Man's Impact on the Environment, Thomas R. Detwyler, ea.. New York: McGraw-Hili, pp. 469-70. 7. Ref.3,p.13. 8. U.S. Fish and Wildlife Service. 1973. Threatened Wildlife of the United States. Bureau of Sport Fisheries and Wildlife, Resource Publ. 114. Washing. ton: U.S. Gov't. Printing Office, p. 35. 9. Ref. 2, p. 35. 10. Ref. 1, p. 131. 1974 .July-August 401 ::::::..e: f,':,':':':': i........ .._.,0..