The average number of archeologic sites along the Colorado River in eastern Grand Canyon between River Miles 65-72 exceeds 12 km-1; the largest concentration from Glen Canyon Dam to the mouth of Grand Canyon. The sites are mostly of Anasazi affiliation, dating from the Pueblo Ito Pueblo II periods (A.D. 800-1200), although older sites of Basketmaker II affinity (about 200 B.C. to A.D. 400) and younger sites of Native American and Anglo affiliation are also present. All of the sites are closely associated with late-Holocene alluvial, debris-flow, and eolian deposits that accumulated in the river corridor during the past 2,500 years. The majority of sites occur on or beneath the surface of ancient alluvial deposits of the Colorado River, which form distinctive high terraces. Lateral shifts of the river extensively eroded these deposits and associated archeologic sites twice in prehistoric times, between A.D.300700 and between A.D. 1200-1400.
In late historic times, numerous sites have been damaged or destroyed by erosion, which has accelerated since 1965-73. The daily operation of Glen Canyon Dam probably did not cause accelerated erosion in eastern Grand Canyon, although the presence of the dam indirectly effects erosion. Generally, sites are eroded by arroyo cutting in the short, ephemeral streams that drain the terraces of the river corridor. These streams are small; 90 percent have catchment area less than 20,000-30,000 m2 and channel length of less than 300-400 m. Driven by excessive rainfall, arroyo cutting deepens, widens, and expands the channel system. The extent of arroyo cutting is related to past and present depositional levels of the river, which are local baselevels of erosion. The post-dam level is 3-4 m below the lowest pre-dam level; this decrease resulted from elimination of the annual flood and a six-fold reduction of sediment load. Eighty percent of tributary streams end above or on the post-dam depositional level, but during large runoff the channels are free to extend upslope as well as downslope toward the river. These channels will eventually extend downslope to the river, where the channel gradient will be lowered 3-4 m. Arroyo cutting will be intensified until channel gradients adjust to the post-dam baselevel.
