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2 Sedimentology and Stratigraphy of the Palisades, Lower Conial\l(h:. and Arroyo Grande Areas of the Colorado River flows above 708 m3/5 (25,OOO ft-~/s) occur under the current operating regime only for isolated experimental purposes. Although seasonal variations in the hydrograph have been Ra(lened. daily discharge Auctuatjons generally occur over a much greater rangt: than in the predam stale (Topping and others, 2003). Control of river discharge by dam operalions hils important implications for the storage and redistribution potential ofsedimcnt;n the river corridor. [11 the absence of flood events, the relatively lillle sediment that the river now carries cannot be deposited at the higher elevations that received sediment regularly prior to dam closure. In the absenee of lower flows (< 142m;/s [5000 ft'/s]). which are no longer allowed to be relensed from the dam under the 1996 Record of Decision signed by the Secrelary of the Inte- rior. (he scdiment~sloragc capability in tht.' main channel is greatly reduced (Topping and others, 201J0a. 2003). Sediment supplied by tributaries below the dam is exported from the canyon on time scales of weeks to months under the present flow-operation regimc (Topping and others, 2000a, b; Rubin and others. 2002.1. Numerous studies have identified consequences arthe altered hydrogr~ph and sediment content in the Colorado River. The size and number of subaerial sand deposits. many of which <.ire used as camping beaches by river nll1- ners, have shown a systemwide decrease over the past fouT decades, punctuated by episodic aggradation during the 1983 flood 01'2,700 m;/s (97,000 ft'/s). the 1996 and 2004 beach/habitat building flow (BHBFI ex peri men IS (1.270 m'/s [45,000 fr;/s] and 1.160 m!/s [41.000 ft'/sJ, respectively). and sediment input from occasional tributary floods (Beus and othcrs. 1985; Schmidt and Grar, 1987; Budhu and Gobin, 1994; Kearsley and olhers. 1994; Kapliuski and olhers, 1995; Schmidt and Leschin, 1995; Wiele and others. 1996; Hazel and olhers. 1999; Schmidl and olhers. 2004). Ripar- ian vegetation has colonized areas at lower elevations than would have been possible in predam time when annual floods removed YOllng vegetation: vegetation encroachment has contributed significantly to the loss of open sand area along the river (for example. Turner and Karpiscak, 1980). The reduction in open sand-bar area has affected recreational use of the river corridor: sand-bar erosion and vegetation growth have altered the biomass composition and riparian habitat in complex ways (for example. Dolan and others, 1977: Caroth- ers and Brown. 1991; Wehb alld olhers. 1999). The biologi- cal communit)' within the postdam river has undergone major changes as a result of colder water temperatures (as subsur- t:1ce re~crvoir water is released from the dam), increased light penetration, and alterations in food sources associated with the low sediment input. Such conditions arc believed to have contributed to the decline in native fish populations in Grand c.:H1YOll. among other effects (Carothers am.! Brown, 1991; Valdez and Ryel. 1995; Douglas and Marsh. 1996). The effecr of Glen Canyon D.:lm operations 011 archaeo- logical resources in Grand Canyon has received little research attention to date. At the time that the Bureau of Recbmation sponsored the creation of the Glen Canyon Environmental Studies (GCES) research initiative in 1982, primary research objectives included physical and biologi- cal resources. while effects on cullUral resources were not addressed (Fairley and others, 1994: Fairley. 2003). The relative inattention paid to potential dam effects on archaeo. logical sites largely stemmed from the perception that. because few archaeological remains were preserved within [he river's annual flood limit. cultural features \.\.'ouJd not be greatly affected by dam operations, It has more recently been sllggesled (Hereford and others, 1993. Yeatts, 1996, 1997; Thompson and POlochnik. 2000) that postdam aitera. tions in the sediment load and flow regime of the Colorado River may significantly affect the preservation potential of archaeological sites within the river corridor, even above the annual flood zone, Of the nearly 500 archaeologieal sites that have been recorded in the river corridor belwl~t'Jl Glen Canyon Dam and Separation Canyon (255 river miles), more than 250 are considered ro be within the Area of Potent.a! Effect (APE) of dam operations designated by the National Park Service (NPS; Fairley and othcrs. 1994; Neal and others. 2000). Repeated monitoring visits have shown that many of the sites located in or on poorly consolidated sediment deposits are actively eroding dll~ to incision by gullies. aeolian deflation processes, and visitor impact (tor example. LCilp and others, 2000; Pederson and others. 2003. Fairley, 2003). The process of gully incision, and the base level to which small, ephem- eral drainage systems respond. v....ere first proposed 10 he linked to dam operations by Hereford and others (1993). In a study of surficial geology and archaeological site l'TOsion. Hereford and others (1993) docllmented increased erosion of predam fluvial terrace deposits! and associated archaeologi- cal sites. by gully incision that accompanied unusually high precipitation in the mid-1970s, Fromlhese observations fol- 100V'ed the hypothesis IIwl rapid rales of incision were caused by the lowering of base level at the mouths of ephemeral drainages to meet the new. postdam elevation of high.flow sedimenl deposition. Hereford and others (1993) proposed that incision rates would remain high until gully morphology has equilibrated to the postdam base level, a~ mlJl.;h as 3-4 III below the lowest predam alluvial terraces. Thompson and Potoehnik (2000) revisited this hypoth- esis in a comparntive study of alluvial terrace evolution in Grand and Cataract Canyons. Thest: authors modified the base-level concept to include the restorative effects of ftU\'ial deposition in the mouths of gullies and arroyos (thus rais~ ing base level). and the potential for aggradation of predam f1uvialtcrmce deposits by aeolian deposition of reworked flood sand. Thompson and Potochnik (2000) concluded that sediment deprivation and lack offfoods. caused by dam operations, have removed the potential for sediment deposition that could ameliorate precipitrttion.induced gully development. They develuped a predictive geomorphic model designed to identify areas prone to gully incision. a concept later modified by Pederson and others (2003), The emphasis of Thompson and POlOchnik (2000) on the poten-