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I I I I I I I I I I I I I I I I I I I 30 en 20 z a: LL LL a a::: w CD :L ~ 10 o 0.30 0.35 0.40 0.45 0.50 0.55 0.60 RATIO OF RIVER WIDTH. W2/WO 0.65 0.70 Figure 4. Ratio of river width in the most tightly constricted part of a rapid to an average unconstricted channel width upstream of the rapid. In the first decade after dam closure, 27 percent of the tributary fans had built outward because of tribu- tary flooding: 10 percent had built outward by more than 49 ft (Howard and Dolan 1976). In the additional 12 years until 1986, severe changes (defined here to involve emplacement of boulders on the order of 3 ft diameter into the main channel occupied when dischar- ges are less than 30,000 cfs) have occurred in enough of the tributary canyons to lead us to believe that, on the time scale of decades, major changes will take place in the rapids. Median, mean, and peak discharges through the Grand Canyon have been significantly altered by the construc- tion and operation of Glen Canyon Dam (Dolan et al. 1974). Maximum powerplant discharges are about 30,000 cfs and, prior to the filling of Lake Powell to oper- ational level, these discharges were generally not exceeded. Only since 1983 have the peak discharges (up to 92,000 cfs) approached the pre-dam annual spring flood levels (80,000 to 125,000 cfs). The large, rare floods that widened the river channel in the vicinity of the debris fans (estimated by Kieffer [1985] to have 25