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. . fish pbpulation is undergoing dras- tic changes in its species composi- tion because of the changing river regime. Many endemic species (squawfish and bony tail chub) adapted to the turbulent, turbid waters are rapidly disappearing (8), and sluggish-water species (bass and baitfish) are now populating the newly created clear-water "tidal" habitat. The river guides have raised a question concerning the long-term effects of the Glen Canyon Dam on the Grand Canyon rapids. As indi- cated earlier, the majority of the 161 Colorado River rapids were created when alluvial fan materials too large to be moved by the parent stream were deposited in the chan- nel by flash flooding of the tribu- taries. These alluvial fans were con- tinually altered by erosion and de- position as long as peak flows on the order of 50,000 cfs occurred (pre-dam); however, deposition of .the large alluvial debris continues and is now unchecked by the bal- ancing erosion of occasional major floods. The guides believe that some of the rapids are becoming more severe and therefore more dangerous. Increased exposure of boulders in the rapids by erosion of sand and small cobbles as well as cementation of boulders by traver- tine may add to navigation prob- lems. Quantification of erosion rates and of the balance between sediment losses and deposition is difficult. Base-line studies have not been made, and there is no systematic measurement program. This data gap can be partially bridged through photogrammetric compari- son of pre-dam and post-dam aerial photography. A detailed compari- son of this type is now being made by the authors. In addition, estab- lishment of several field sites for continual monitoring of long-term changes in riverfront morphology and vegetation is expected within a year. Man's use of the Colorado Although hundreds of thousands of people visit the Grand Canyon's rims yearly, very few hikers reach the Canyon floor, and, until recent- ly, even fewer people made boat t,,> . ~"~'. ~~._- ~....::..,-,.. ~ '..-2::.._Lr~ ~~~, Figure 7. Beaches in the narrow Granite Gorge section of the river are small, infre- quent, and very susceptible to erosion, which exposes the underlying coarse rock debris, making camping and mooring diffi- cult. (Photo by Alan Howard.) Figure 8. A comparison of Trail Canyon beach at Mile 219 under pre-dam conditions in 1932 (top) and post-darn conditions in 1969 (bottom) shows pronounced erosion. (1932 photo by .John H. Maxson; 1969 photo by Art Gallenson.) .f. "J!... :i&"RT.l... ,.. ..9J'!'!..........I'...... ..... .. ... .... '.... ."1".". ".-...."... ..... .".:~. . . j ,..~. . y, ~.'~~~ ..'. '.... '"" .'-.. ;'.~" N.~ .". I" . h_.. ~ '$ ~". . ......~.. ",~ ~. . . \...c "" ~-__ . ....1":">~..~.r. ~'- u. 1974 .July-August 399