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4 11M 10, O'CONNOR lOT AL, The Stratigraphic Record of Colorado River Floods in the Grand Canyon Belore construction of Glen Canyon Dam, Colo- rado River floods in the Grand Canyon had high suspended load concentrations, on the order of 2- 3% by weight le,g" Howard 1947), In regions 01 flow separation during floods, such as eddies and backwater areas, local velocities are 1-15% of those in the main thread of flow, and suspended sand and silt rapidly settles (McKee 1938; Schmidt 19901, Protected sites preserve sequences of these line-grained flood deposits, forming a stratigraphic record of the largest floods (Patton and Dibble 1982; Kochel and Baker 1982, 1988; Webb et al, 1988b), We have found several such deposits be- tween Glen Canyon Dam and the Grand Canyon streamflow gage, One remarkable site, which we have infonruilly named Axehandle Alcove, is 3 km downstream from the gage at Lees Ferry. Axehan- die Alcove is a small rock sheltc:r within a 50 m high canyon wall of Kaibab Limestone (figure 1), Large floods inundated this alcove, leaving behind successive layers of grey silt and fine sand that, Figure 3. Schematic representation of two measur<:d sectiODS at Me- handle Alcove showing flood units, locations and results of dated sam. pies of organic material, and a stage- 71 discharge relation based on step- backwater flow modeling, The elevation datum is arbitnlry. Colo- rado River flood deposits were dis- criminated from loca1ly derived (non-flood) sediment on the basis of sedimentology and color, Daahed lines connect the tops of deposi- tional units that could be traced from one section to the other, The highest Gl deposits were traced to . level about one meter higher than the top of the stratigraphic sections, Elevation (meters) 70- 69= 68- until recently, have remained relatively undis- turbed, Stratigraphy ad Chronology of Dep08im at AzmiZIJ- die Alcove, Recent gullies have incised this ssdi- . Qlent in Axehandle Alcove and exposed deposits of at least 15 floods that had discharges comparable to those 01 the large historic flows 011921 and 1884 (figures 2 and 31, The stratigraphic sequence at Axehandle Alcove is capped by a thin, grey, silty sand layer I G 1) that can be traced for several hun- dred meters along the canyon margin (figure 41, ,Based on the elevation, fresh appearance, and con- tinuity of Gl, we speculate that it was emplaced by the =8500 m3sec-1 flood of 1884, In addition, at several places along both canyon walls, pockets 01 fine organic detritus in a matrix of grey, lami- nated silt and fine sand have been preserved in small crevices up to 4 m above the highest expo- sure 01 G 1. These crevice deposits are the highest evidence of Holocene flooding found in this reach of the Colorado River, At Axehandle Alcove, deposits from Colorado River floods are separated by layers of red coarse sand and gravel derived from local drainage of adja- Stratigraphy Discharge Rating m3sec" Gl --- - 8000 --- --- - 7500 GX.I6052 2340-1994 GX.1S053 - 7000 2307-2062 GX.16054 - 6500 3916-3704 - 6000 -5500 _ Colorado River flood dsposit c:J Locally-derived sediment ""'" Heerth ] Radiocarbon Sample Locations Laboratory Sample Number ~,. Charcoal Accumulation Age, cal yr BP