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Figure 9. On a terrace deposited in 1973 as a result of flooding of the Little Colorado River, the 3-foot-thick deposits (vertical bank) partially buried the beachfront cot- tonwood (right) and willow (left). Subse- trips down the Colorado. During the mid-1960s, however, when Mar- ble Canyon and Grand Canyon were threatened by plans for two new hydroelectric dams, the battle between conservationists and the Bureau of Reclamation received na- tional publicity, and before a ten- year moratorium was placed on the project, thousands hastened to travel through the Grand Canyon "before dam builders ruined it." Many celebrities also rushed to ride the rapids, providing additional publicity. By 1969, the Grand Can- yon river trip had become one of the world's best-known white-water river trips,. described by some as the finest wilderness experience in North America. With the exception of a few Na- tional Park Service patrol boats and a very limited number of pri- vate white-water boats, the Grand Canyon float trips are conducted by a dozen or so commercial outfitters. The commercial boats are of two types: the smaller rowing rigs carry 400 American Scientist, Volume 62 quent lateral erosion has reexposed many of these plants' lower branches. The cotton- wood, willow, and tamarisk are adapted to such episodes of alternating burial and ero- sion. (Photo by Alan Howard.) from two to twelve passengers with a crew of one or two, and the larger ou tboard - motor- powered rub ber boats can accommodate betw3en seventeen and twenty passengers and a crew of two (see Fig. 3). The outfitters usually run two or more boats together, for a party of forty passengers, the maximum permit- ted by the National Park Service. These people remain together throughout the five- to eighteen-day trip. The outfitters and their passengers must use one of the river bars, or beaches, for camping each night, and most parties stop at one of the beaches for lunch. This means that, at the more desirable campsites, between thirty and forty people may use the beaches each summer evening. Human impact includes occasional litter, burial of chemi- cally treated waste, and the direct stress associated with people walk- ing on the vegetation and unstable sedimentary deposits. Effects of this usage are summarized in Table 2. In the summer of 1973, our research team stopped at more than 100 campsites specifically selected to represent examples of the various beach types. We were unable to de- tect any significant degradation of the beaches. and adjacent areas be- cause of littering or waste disposal. National Park Service regulations require commercial outfitters to carry chemical toilets and to leave the beaches as clean as possible. Most comply and haul their trash and waste out of the canyon; how- ever, the direct impact of thirty to forty people conducting the normal bathing, cooking, and camping rou- tine was found to be significant. We noted that footpaths on some beaches had resulted in surface ero. sion of up to two feet, and a one- foot reduction was common. In ad- dition, heavy foot traffic roughens surfaces that are periodically in- undated, contributing to accelerat- ed sediment losses during the fol- lowing diurnal high water. Some conservationists are con- vinced that this heavy commercial use is leading to irreversible erosion and even more rapid degradation of the river's deposits. In addition to the concern about erosion trends and ecological changes, the Nation- al Park Service is equally worried about overall degradation of the Grand Canyon "wilderness experi- ence." Outboard motors are one of the major contributors to noise pol- lution. Also, since all the outfitters usually schedule their trips to coin- cide with the high-water periods, the more scenic stops tend to be congested and overused. Most of the outfitters disagree with this viewpoint, as one would expect, and, in fact, a recent study by Bos- ter (9) shows that most people making river trips do not consider crowding to be a serious problem. The future It is a much simpler task to list the environmental problems along the post-dam Colorado than it is to .recommend specific solutions. It appears unlikely that anything can be done to increase the sediment yield or the flood stages of the upper Colorado River as long as the Glen Canyon Dam is in place. Di- version of waters from the upper reaches of the system to areas below the dam has been suggested as on this comn seem! TherE zatiOl enVlr' and 1 cal I way I The i the longe this the] men1 do t) gatie time Serv decil man Impl men dam lian a ce the to tl use cial wild -it Sine vice nve tors the sub nes: Su ThE stal 80,l 0.3: da~ Th of 1 po~ she na' In by wa bie Da 4,C m~ Tr ex re~ WI