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1 JIM E, O'CONNOR ET AL. UTAH A-iiZoNA. -. - --. -. - - -. Glen ClInyon Dam --/" Axehsndfs Alcovs ~ iA. SUwmgllllnglllllan CoIoIIdII_ IIlN1 Forry have simply stated that the time scales of adjust- ment ue too long to evaluate adequately with the present data (Howud and Dolan 19811. To provide a fuller picture of the incidence of luge Colorado River floods, we report a 4500 yr stratigraphic rec- ord of large floods in the upper Grand Canyon near Lees Ferry, Arizona, This is one of the longest such. records available for any major U ,S, river, and it should be relevant to studies of Colorado River hy- drology, geomorphology, and ecology, as well as Colorado River basin paleoclimatology, We have combined this stratigraphic record of prehistoric floods with the gaged record of 20th century floods to provide quantitative estimates of past recur- rence of large floods, The Gaged Record of Colorado River Floods in the Grand Canyon, Arizona Where the Colorado River enters Grand Canyon National Park, near Lees Ferry, Arizona (figure 1), stages of annual Colorado River floods have been recorded by a staff gage established in May, 1921 and, since January 1923, at a U.S, Geological Sur- 114- 113" :J1" ARIZONA - ~- Coilndl_ IIYlI1II 36" vey (USGS) recording streamflow gage (Colorado River at Lees Ferry\ {figure 2\, A permanent stream- flow gage was also established 150 Ian downstream (Colorado River near Grand Canyon) in October .1922, These two gages provide the best instro- mented record of large Colorado River flows in the Grand Canyon, The largest flow considered part of the. gaged record at Lees Ferry was on June 19, 1921, which had an estimated discharge of 6250 m3sec -I based on extrapolation of a stage-discharge rating curve to highwater marks at the Grand Can- yon gage (Dickinson 1944, p, 174 and 190; USGS 1973, p, 6111, For these two gages, the maximum discharge known outside the period of record was about 8500 m3sec-1 on July 7, 1884, According to E. C, LaRue (1925), during this flood, a resident of Lees Ferry rescued bis cat from the branches of an apple tree, Decades later, the resident, with "the height of the water on the trunk of the tree , , . well impressed on his mind," assisted a surveyor in referencing that elevation to the datum of the stream gage established at Lees Ferry in 1923, The 8500 m3scc-1 discharge was estimated by extrapo- lating the Lees Ferry stage-rating relation above the stage associated with a measured discharge of 3400 112" 111- snam ~ing slllion Colorado RIYor II Gllnd Canyon 50 Kilometers I o I Figure 1. Location of study area and USGS Colorado River streamflow gaging stations referred to in text,