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4 Sedimentology and Stratigraphy of the Palisades, lower Comanche, and Arroyo Grande Areas of the Colorado River expanded upon in this present study. Radiocarbon dating work (Hereford. 1993; Hereford ond olhers, 1996) has also provided much of the available infonnation on the timing of variolls alluvial terraces and debris-fan deposits at Palisades. It is noteworthy that the Palisades area was one location used by Hercford and others ( 1993) 10 outline the base- level hypothesis discLlssed above. Thompson and Potochnik (2000) studied the Palisades area during the formulation of their restorative base-level hypothesis. and they concluded that drainage catchment areas at Palisades did support the idea that incision of alluvial terraces could be attributed to dam-induced lowering of base level. Thompson and Potoch- nik (2000) list Palisades and Arroyo Grande as two arcas where, during their geomorphic survey and modeling study. they identified stratigraphic evidence for paleogullies that had filled with aeolian sand. Grams and Schmidt (1999) used ground (oblique) and aerial photography of the Palisades area. among other study sites, to evaluate changes in the extent of surficial sand deposits since 1890, Repeated high-resolution mapping by Yea"s (1996. 1997) and Hazel and olhers (2000) demon- strated erosion and aggradation of sand deposits at Palisades as a result of the 1996 BHBF experimenl. Notobly, Yealls (1997) and Hazel and others (2000) documented the migra- tion of sediment deposited in the main arroyo mouth at Pali- sades to higher elevation over the year following the flood. atlribwed this migration 10 ::leolian processes, and identified such consequences of the BHBF as potentially beneficial for preservation of cultural resoLlrces above the 1,270-mJ!s (45.000-ft!/s) level. Gellis (1994) reported ficld observations of erosion at archaeological sites during work with the Zuni Tribe, including parts of the Palisades and Lower Comanche areas discll~sed here. Pederson and others (2003) monitored arroyo incision at both Palisades and Arroyo Grande, among other siles, in a geomorphic study of gullying processes and an assessment of the etTectiveness of erosion-control struc- tures sllch as checkdams. Mapping of surficial deposits at Granite Park (arrro:~j- mately 2 km downstream of the Arroyo Grande site dis- cussedhere) by Hereford and others (2000) pro'vides- information applicable to geomorphic interpretations at Arroyo Grande. Several rndiocarboll dates listed in Hereford and others (2000) were for samples from stratigraphic units at Arroyo Grande. aiding age determination of deposits in that area. Fairley and others (1994) completed the first compre- hensive survey of archaeological sites along the river cor- ridor in Grand Canyon. This survey provided baseline data fl.)r uefining rhe depositional confext of man)' archaeological sites. and it generated the first data set against which later monitoring surveys could be compared. Subsequent monitor- ing sllmmaries by NPS have provided repeated geomorphic observations related to archaeological site location. condi- tion. and preservation (Coder and others, 1994a, b, 1995; Leap and others, 1996. 1997, 1998.2000.2003; Leap and Kunde. 2000; Dierkcr and others. 2001. 2002), Pederson and , others (2003) used repeated aerial surveys to evaluate rates of gully irti~io~ in areas known to be archaeologically sig- nificant, includihg Arroyo Grande. Sedimenlology and stratigraphy of flood dcposits in Grand Canyon have been addressed by several previous stud- ies. McKee (1938) fir$1 presented facies descriptions ofCol- orado River flood strata at a number of exposures fit tribut3ry mouths in the canyon. Rubin and others (1990) and Schmidl (1990) used stratigraphic exposures in active river-level sand bars to describe the formation and evolution of separa- tion and reattachment bars in zones of flow recirculation in eddies, Rubin and othcrs (1994) analyzed stratigraphy within deposits from the 19832.700 Ill'/S (97.00U ft'/s) flood and weaker flood events in 1984-1986. using the data to estimate deposition rates and to evaluate the potential impact of vari- ous dam-controlled flow regimes on the erosion and accumu- lation of sediment on camping beaches. O'Connor and others (1994) used paleoflood deposils, doted by radiocarbon tech- niques, to estimate discharge levels for floods as old as 4,500 years BP. The peak discharges of the paleofloods studied by O'Connor and olhers (1994) were subsequently revised by Topping and olhers (2003) using archived US Geological Survey gaging data that were not readily available for previ- ous analysis. Grain-size analyses of sediment sampled from flood deposils in Glen Canyon (below the dam) and Grand Canyon have demonstrated sediment-supply limitation in both predam and postdam flood events (Rubin and others, 1998; Topping and olhers, 2000a. b; see also Rubin and Topping. 2001). Supply limitation in flood deposils was shown hy those studies to be manifested by a ~oarsenjng upvv'(1rd of grain size within individual flood deposits, in contrast to the ti.ning-coarsening-tlning or fining-upward fluvial sequences that result from supply-unlimited flows. Upward coarsening of deposits from supply-limited floods occurs as suspended sediment becomes progressively depleted of fine-grained material in a supply-limited system (the finer sizes having been transported downstream first and farthesl). Fine sedi- ment can then be winnowed from the stream bed during the later stages of the flood, increasing the median grain ~ize of the sediment that remains exposed at the bed (Rubin and olhers, 1998). The relarionships between sediment- supply limitation in the Colorado River and the resulting sedimentology of flood deposits disclIssed by Rubin and others (1998) and Topping and others (2000a, b) are further developed in this study of predam fluvial sequences. Methods Data collected from the tlm~e sires discllssed in this report were obtained primarily through detailed examination of vertical stratigraphiL' s~cliuns. At all three locations, the presence of arroyo networks facilitated the exposure of verti. cui t:1ces for this purpose; at Palisades and Lower Comanche, several additional sections were exposed by digging small