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01.296 sand bars. while not noticing other aspects. such as changes in wildlife populations. These observations must be taken in aggregate to provide an accurate accounting of environmental changes in Grand Canyon. Much has changed along the Colorado River since most of the Old Timers and other historic river runners were in Grand Canyon. The following changes. many of which are supported using photographic or other evidence, are probably significant: . The low waler temperature, increase in riparian vegetation, lack of substantial sediment in the river, noise from aircraft, and deteriorated air quality are considered the largest changes in Grand Canyon. The largest changes in the river corridor. other than rapids. were observed just downstream from Nankoweap Creek (mile 52.2-R), and at the mouth of the Little Colorado River (mile 61-L). Elves Chasm (mile 116.5-L). and Tapeats Creek (mile 133.8-R). . The Old Timers saw little evidence of debris fiows before closure of Glen Canyon Dam. The frequency of debris fiows may have increased after the early 1960s. as speculated by Melis and others (1994) and Webb (]996). Certain rapids -- such as Crystal (mile 98.3). Bedrock (mile 130.5). and House Rock (mile 16.9) -- are now more difficullthan in the pre-dam aa. Lava Falls Rapid (mile 179.5), which was always large, also was discussed as having changed. Other rapids are unchanged or are less difficull. . Erosion of camping beaches used by pre-dam river runners is severe. Now. some formerly popular camping sites are not easily used for camping. . Fires had a significant impact on the amount of driftwood along the river corridor. Fires were set randomly and specifically at certain popular places. particularly the mouth of Nankoweap Creek (mile 52.2-R) and Tanner Canyon (mile 68.5-L). . Riparian vegetation. mostly tamarisk. is much denser along the river. whereas cottonwoods and tree-form willows decreased in the post-dam era. Tamarisk slowly increased, particularly in the I 940s and 1950s. but accelerated after 1963. . Perennial marsh vegetation was not observed or photographed along the pre-dam river except near springs. . Catfish were the most common fish caught in the pre-dam river, allhough rainbow trout were also present. Some of the Old Timers caught native fish, mostly befNe about 1950, and tbe species they observed could not be accurately determined unless the catch was photographed. . Ducks and other water birds were commonly observed along the unregulated river, but they were not as abundant as they are now. in accord with the findings of Slevens and others (1997). Other changes mayor may not be significant. if in fact changes even occurred: (I) Bats and some species of birds may have decreased along the river. whereas swallows may have increased. (2) Bighorn sheep possibly are more numerous; they certainly are observed more often now. No changes can be inferred in the population of deer or other large mammals along the river. (3) It is uncertain whether the population of beavers has changed. and otters remain an uncommon sighting. (4) Bothersome insects were common before Glen Canyon Dam. although it is unknown if species or abundances may have changed. Some of the issues raised in this report have significant implications for dam manageml?nt. The increase in frequency of debris flows will hasten a long-postulated change in the longitudinal profile of the river; without dam releases significantly higher than power plant capacity, the river will eventually attain an exaggerated pool-drop configuration. ]mplications of such a trend might be that navigation of rapids becomes generally more difficult, and in some cases. the river may become impassable. Continued aggradation of the river channel by coarse sediment delivered during debris fiows will likely fill-in deeper pools above and below rapids. while also enhancing the size of eddies. On the basis of photographic evidence and sedimenl-lransport studies, bars are predicted to continue decreasing in size (Laursen and others, 1976). Tamarisk was increasing despite pre-dam fiooding. Although dam releases could be used to thin some of the population. tamarisk would have become a common, although exotic. resident of Grand Canyon even if the dam had not been buill. Marshes are an artifact of fiow regulation, not a natural feature of the pre-dam environment (Webb, 1996). Native fish may CONCLUSIONS 31