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(jvG~7 - Modelling 01 Sdnd deposition in Colorado Ri'Ver )65 . Debris flows and floods from streams in side canyons (Howard and Dolan. 1981; Schmidt. 1987; Webb nal., 1989; Schmidt and Grar. 1990; Metis no/.. 1994; Schmidt and Rubin. 1995) form debris fans that partially constrict the channel. and recirculation lones are generated in the lee of the channel constriction. The spacings between debris fans ace controlled 10 a large degree by bedrock structure (Dolan tt al., 1978). The bed of the Colorado River in Grand Canyon National Park is about 600/. bedrock. talus blocks or boulders. Sand supplied to the mainstem river by tributary floods is stored primarily along the channel sides and in recirculation lones. Sand is slored temporarily on tbe cbannel bonom (Howard and Dolan. 1981; Wilson. 1986; Schmidt and Gear. 1990). The four study reaches discussed in this report are located between 112 and l40km below the dam. starting about 6k.m downstream from the confluence with the Little Colorado River (Figure 14.1), one of the two main sand-supplying tributaries. The reaches modelled in previous studies tended to have narrow con- strictions with large, abrupt expansions that produced large, weU-developed recircu- lation lones. These large zones can be effective at storing large volumes of sand. Reaches modelled in this study have channel constrictions aod recirculation lones, but the expansions are narrower or more constrained and the resulting recirculation zones are smaller than the pre\iously modelled reaches. In the two downstream reaches in this study. tbe recirculation-zone sand deposits are less significant than the channel-margin deposits. To clanfy the discussion and to avoid repeated. lengthy descriptions of morphological featurcs in the reaches, these features are designated "'itb letters in the accompanying figures. . .. " , .~"--- .- , .~ 'C . Figurr '4.' .Aerial pholos 01 study Sites and map shoINing S1udy Sire-locations. . --