Map Showing Surficial Geology and Geomorphology of the Palisades Creek Archeologic Area, Grand Canyon National Park, Arizona

Hereford, Richard

The mapped area lies mostly downstream of River Mile 65.5 along the Colorado River in the eastern Grand Canyon, which is 65.5 miles downstream of Lees Ferry, Ariz. The geologic mapping and related investigations were undertaken to determine the late Holocene history of erosion and deposition along the Colorado River and to determine how this erosion damages archeologic sites. Archeologic sites are abundant along the Colorado River between River Miles 65-72, averaging more than 5 sites/km, which is the largest concentration of sites in Grand Canyon. The sites are mostly of Anasazi affiliation, dating from the Pueblo I and II periods (A.D. 800-1200), although younger sites of Native American and Anglo affiliation are also present (Hereford and others, 1993). The geologic mapping was directed at determining how the ongoing erosion of terraces and archeologic remains is affected by regulated streamflow, which began in 1963 with the closure of Glen Canyon Dam, 131 km upstream of the mapped area. In addition, the map was made to provide baseline topographic and geologic information against which future changes resulting from regulated streamflow or natural conditions can be judged. The work was part of the Glen Canyon Environmental Studies in cooperation with the U.S. Bureau of Reclamation. Regulated streamflow, defined here as the water and sediment discharge regimen of the Colorado River since 1963, is substantially reduced in sediment load, sediment concentration, duration of high flows, and peak-flow rates compared with the unregulated streamflow of the pre-dam era. In the present discharge regimen, sediment load has been reduced by a factor of six and the annual flood, which was the principal agent of geologic change, has been eliminated. It is almost certain that changes of this magnitude are not possible under natural conditions (Hereford and others, 1993). Surficial geologic processes along the river are dominated by the interaction between the Colorado River and tributary streams. This interaction has been changed by the long-term sediment depletion and elimination of the annual flood. The capacity of the river to remove boulder-size sediment carried to the main channel by Palisades Creek is greatly reduced, and the amount and areal extent of sand deposition by the river is also substantially reduced. Reduced 1 transport capacity of the river will lead to constriction of the main channel by tributary-stream sediment transported mainly by debris flows originating in the Palisades Creek drainage basin. The reduced concentration of sand-sized sediment in the river has probably altered the erosional balance of the river corridor; this change increases erosion by the small, ephemeral streams that drain the corridor. The large-scale topographic base map was made to show the geologic and geomorphic elements of the river corridor, which are too small to show on standard U.S. Geological Survey 1:24,000-scale topographic maps. The 1:2,000-scale base map was produced photogrammetrically using low-altitude aerial photographs mounted in a stereo analytical plotter. A ground survey was done in 1990 to rectify the aerial photographs; vertical control and latitude and longitude were obtained from maps and field measurements made by Lucchitta (1991). The coordinates of the base map differ slightly from the map of Lucchitta (1991), which should be used for critical measurements of latitude and longitude.

Map Showing Surficial Geology and Geomorphology of the Palisades Creek Archeologic Area, Grand Canyon National Park, Arizona

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