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equilibrium" (Langbein and Leopold, 1964) that likely existed between the river and its tributaries in the pre-dam era have been perturbed in favor of the tributaries since 1963. Anticipated changes of locally-increased flow gradients and navigational hazards in rapids are only the most obvious consequences of continuing debris flows. Other consequences include burial and erosion of existing sand bars, increased sand storage in eddies and pools, and overall aggradation of the channel. TRIBUTARIES AND REACHES OF THE COLORADO RIVER IN GRAND CANYON Definition of Geomorphically Significant Tributaries Our definition of "geomorphically significant tributaries" includes numerous small drainages that have potential to produce debris flows that alter the river channel or yield significant amounts of streamflow-transported sediment (appendix I), The criteria for designating drainages were determined from analysis of 44 U,S. Geological Survey 7.5- minute topographic maps of the river corridor and 126 maps of the Grand Canyon region. Included were all tributaries between Glen Canyon Dam and the Grand Wash Cliffs (fig. I) that: I) have drainage areas larger than 0.01 krn2, 2) have mapped perennial or ephemeral streams, 3) clearly terminate at the Colorado River in a single channel, and (or) 4) contribute to formation of obvious debris fans and (or) rapids, We excluded the Paria and Little Colorado Rivers and Kanab and Havasu Creeks because their sediment yields are relatively well known. Drainage areas that could not be designated significant using the criteria outlined above were designated as "extra areas", rather than tributaries. This type of drainage area consists of steep slopes with no identifiable channel on topographic maps and contributes sediment primarily by streamflow, though it may occasionally yield small debris flows. Drainage areas of geomorphically significant tributaries and extra areas were digitized from hand-drawn outlines on 7.5-minute topographic maps. Numerical routines that define drainage areas from digital elevation models (Jensen and Domingue, 1988) performed inadequately in the steep terrain of Grand Canyon and were not used, Place names were derived from well-known river guides (Stevens, 1990), gazetteers (Brian, 1992), and topographic maps, Exceptions were made where Board of Geographic Names spellings are not consistent with the person being commem- orated. For example, "Deubendorf Rapid," the official name, is incorrectly named for Seymour Dubendorff (Webb, 1996), and therefore we refer to this rapid as DubendorffRapid (appendix I). Designation of Reaches of the Colorado River Researchers long have recognized differences in the characteristics of the river corridor through Grand Canyon (Howard and Dolan, 1981), Previous studies have divided the Colorado River between Lee's Ferry and Diamond Creek into eleven reaches on the basis of topographic characteristics and bedrock type (Howard and Dolan, 1981), as well as on the basis of individual formations at river level and channel gradient (Schmidt and Graf, 1990). Using an approach modified after Howard and Dolan (1981), Melis modi fied Schmidt and Graf s reaches by including data on debris-fan morphology and spacing, By this approach, Melis (1997) re-defined the eleven reaches of Schmidt and Graf (1990) into six major reaches with two subdivisions of Reach 4 (table I), For this paper, we add a reach 0 (Glen Canyon) and extend reach 6 of Melis (1997) to the Grand Wash Cliffs at the downstream end of Grand Canyon (table I). This designation of geomorphic reaches differs significantly from the reaches required for a sediment mass balance between gaging stations on the Colorado River and tributaries with sediment- yield data. At various times, sediment data have been collected at gaging stations on the mainstem Colorado River and its tributaries (Garrett and others, 1993; Rote and others, 1997). The mainstem gaging stations are the Colorado River at Lee's Ferry (station 09380000, mile 0,0), the Colorado River above the Little Colorado River (09383100, mile 61.5), the Colorado River near Grand Canyon 6 Sediment Delivery by Ungeged Trlbuterles of the Coloredo River In GrInd Csnyon