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.42 Sedimentology and Stratigraphy of the Palisades, lower Comanche, and Arroyo Grande Areas of the Colorado River Corridor, Grand Canyon, Arizona f I ~ ,I I: By Amy E. Draut', David M. Rubin', Jennifer L. Dierker', Helen C. Fairley', Ronald E. Griffiths', Joseph E. Hazel, Jr', Ralph E. Hunter', Keith Kohl', Lisa M. Leap', Fred L. Nials', David J. Topping" and Michael Yeatts' Abstract This report presents initial results of a joint effort between geologists and archaeologists to evaluate the sig- nificance of various depositional proccss~s and environ- ments in the prehistoric formation and n"l.odern preservation of arch<teological sites along the Colorado River corridor in Grand Canyon National Park. Stratigraphic investigations of the Palisades. Lower Comanche. and Arroyo Grande areas (lfGrand Canyon yield detailed information regarding the sedimentary history at these locations. Reconstruction of past depositional settings is critical to a thorough undcrstanding of the geomorphic and stratigraphic evolution of these three archaeo]ogically significant areas. This examination of past sedimentary environment:; allows the relative significance of fluvial. aeolian. debris-fan. and slope-wash sedimental)' deposits to be identified at each site. In genernl the propor- tion of fluvial sediment (number and thickness offtood deposits) is shown to decrease away from the rivcr. and locally derived sediment becomes more significant. Flood sequences often occur as 'couplets' that contain a fluvial deposit overlain by an interflood unit that reflects re\\.:ork. ing offluvia! sediment at the land surface by \I,'ind and local nmotT. Archaeological features are built on and buried by sediment of various depositional environments. implying a complex interaction between geologic and cultural his- tory. Such field analysis, \,....hich combines geological and I u.s. Geoll)glCLll Sul"\cy. 400 Natural Bridges Drive, Santu CnJz, CA 9501'10 ~ Naliona] Park Semcl", H23 San Fran":lS';O SI.. Suile B.. Flagsmll AZ RhOOI t U S. G('ological Sul"\'cy. Grand Canyon Moniroring <lnd Rc~earch C.:nter, 2255 N Gemini Drive. Flagstuff. AZ S6001 , O('partl11ent of Geology, Nonhem ArlZOIHl Ulli\'('r~lly, Flags.tafT, AZ 86011 'GcoAreh 10450 W. ~th Plact". Lakewood, CO 80215 ^ Hopi Cu[;uml Pres('r\"<Itlon Office. Northern Ariwn3 Uni\"ersll)' Depart- ment of Anthropology. Flugstall AZ X601 t archaeological information and techniques, can provide a basis for future determination of the effects of Glen Canyon Dam operations on selected areas of the river corridor. This knowledge is essential to the development of preservation strategies for cultural resources in Grand Canyon. Introduction Since the closure of Glen Canyon Dam in 1963, drastic changes in the natura] hydrologic and sedimentologic regime have resulted along the Colorado River environment in Grand Canyon (see. for example, Andrews. 19R6; Webb and others. I Q9Q). The dam has reduced the supply of sediment at the upstream boundary of Grand Canyon National Park by about 95 percent. with the Paria River remaining the only major supplier of sediment at the upstream park boundary. The predam mean annual seJlIl1em load at the up-stro;;:am park boundary \\"as -60 million metric tons: the postdam mean annual sediment load is -3 million metric tons (Topping and others. 2000a). The other substantial postdam supplier of sediment is the Little Colorado River. which enters the Colo~ rado 98 km downstream from the Paria River and supplies an additional ;-10 percent of the predam sediment load (Top- ping and others. 20003). Operation of the dam smoothes natural variations in the river's hydrograph that occurred in response to snow melt and other seasonal climatic fluctuations. Predam discharge records from the Colorado River gage at Lees Ferry, Arizona (US Geological Survey station number 09380000). 24.5 km below Glen Canyon Dam, indicate that discharge was typi- cally below 226 Ollis (7.980 ft'/s) for approximately half of the year. while the mean annLlal spring snow-melt flood was 2,410 m'/s (85,000 ti'ls: Topping and others, 20(3). The discharge range released from the dam is substantially less than this natural variability; maXinllll'll power-pl~lnt capacity calls for flows no greater than 940 m'ls (33.200 W/s), and