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to be false, the peak discharges of these floods were revised upward by Gatewood and Hunter in 1938 to 300,000 and 220,000 ft3/s, respectively. The analyses in this paper of the raw data from Lees Ferry (which accounts for the progressive development of backwatered flow conditions in the reach at high stages), of the surveys of high-water marks at the Grand Canyon gaging station and at River-mile 233.7, and of the published data from other upstream and downstream gaging stations, indicate that the most likely peak discharges of the 1884 and 1921 floods at Lees Ferry were 21O,000:!:30,000 ft3/s and 170,000:!:20,000 ft3/s, values close to those originally estimated by the USGS. The progressive flattening of the water-surface profile at high stages and the consequent downward revision in the peak discharges of these two floods imply that the peak discharges in the 4,500-year paleoflood series of O'Connor and others (1994) need to be revised, The peak discharges of many of the prehistoric floods associated with these paleoflood deposits were, in fact, comparable to the peak discharges of floods during the early part of the 20th century. This observation leads to a minimum reduction of a factor of two in the return periods for these floods estimated by O'Connor and others (1994). Analyses of the 1921-2000 continuous record of instantaneous discharge provide insight into the pre-dam natural variability in the discharge of the Colorado River, and the hydrologic changes imposed on the Colorado River in Grand Canyon National Park by the construction and operation of Glen Canyon Dam. The continuous record of instantaneous discharge can be either requested from the USGS Grand Canyon Monitoring and Research Center, Flagstaff, Arizona, or obtained electronically at http://www,gcmrc,gov. Prior to the closure of Glen Canyon Dam in March 1963, the discharge of the Colorado River at Lees Ferry varied substantially over decadal time scales, The median discharge during the driest decade, the 1930s (6,720 ft3/s), differed from the median discharge during the wettest decade, the 1920s (10,700 ft3/s), by a factor of 1.6, The median discharge for the entire pre-dam period from May 8, 1921, through March 12, 1963, was 7,980 ft3/s. Although large floods were common in the pre-dam river, discharges exceeded 18,500 ft3/s only about 25 percent of the time, Topping and others (2000) showed that sand accumulated substantially in the pre-dam river in Marble and upper Grand Canyons only when the discharge was lower than about 9,000 ft3/s, and that sand eroded from Marble and upper Grand Canyons when the discharge was higher than about 16,000 ft3/s, The pre-dam decadal variation in discharge therefore had major implications for sand storage in Marble and upper Grand Canyons; the 1920s were likely the pre-dam decade most dominated by erosion of sand from this reach, and the 1930s were likely the pre-dam decade most dominated by accumulation of sand in this reach, During the pre-dam era, the discharge of the river was fairly steady over sub-daily time scales, with a median daily rauge in discharge of only 542 ft3/s. Therefore, daily mean discharges provide a reasonable characterization of the hydrology of the pre-dam river. An exception is the summer thunderstorm season, when the daily range in discharge was observed to exceed 30,000 ft3/s, but these extreme daily ranges occurred on average only dUling I day in every 3 years. Daily ranges in discharge were larger than 10,000 ft3/s only during I percent of all pre-dam days. The median daily range in discharge was greatest during the June part of the snowmelt flood (3,230 ft3/s). The decadal daily range in discharge was somewhat correlated with the decadal discharge. As in the case of discharge, the median daily range in discharge during the dtiest decade, the 19305 (516 ft3/s), differed from the median discharge during the wettest decade, the 1920s (808 ft3/s), by a factor of 1.6. The operation of Glen Canyon Dam has removed the seasonality from discharge and from the daily range in discharge. Dam operations have removed flood flows and base flows, and dam operations for hydroelectric power generation have introduced wide-ranging daily fluctuations in discharge that are relatively consistent throughout the year, The changes that dam operations have imposed on the hydrology of the Colorado River at Lees Ferry exceed anything in the quasi-natural pre-dam period of record. As for the pre-dam tiver, discharge and the daily range in discharge have varied substantially over decadal time scales in the post -dam river as dam operations have evolved in response to the filling of Lake Powell reservoir and to growing concern about the effects of dam operations on the downstream Colorado River ecosystem. From the 1960s to the 1990s, the median discharge of the Colorado River at Lees Ferry increased from 9.490 ft3/s to 13,500 ft3/s. During the wettest post-dam decade, the 1980s, the median discharge of the Colorado River at Lees Ferry was 15,900 ft3/s, only slightly higher than that during the 1990s. During the four post-dam decades, discharges under which sand can accumulate in Marble and upper Grand Canyons have progressively disappeared. During the 1960s, discharge was greater than 9,000 ft3/s 52,7 percent of the time; during the 1970s, 02223 Conclusions 57