LONGITUDINAL AND TEMPORAL VARIATIONS IN SAND BAR SIZE ALONG THE COLORADO RIVER IN GRAND CANYON

There is significant reach-scale variability in the size distribution of eddy sand bars among different reaches of the Colorado River in Grand Canyon. GIS analysis of data based on mapping of aerial photographs from ten different dates between 1935 and 1996 was used to develop a sufficiently large data set so that this variability could be quantified. Despite wide variations in sand bar size from site to site, general trends at a reach scale are detectable. Monitoring of eddy sand bars as primary sites of storage for sand is important in part because of their role in life cycles of endemic fish species, as substrate for riparian vegetation, and as campsites for recreational boaters. The extent and appearance of sand bars are considered one measure by which the regulated system can be compared to the natural system prior to closure of Glen Canyon Dam in 1963. Some studies have documented a reduction in the size and number of sand bars since dam closure. Our data indicate that sand bars in a 14-km reach near Lees Ferry, located 25 km from the dam, behave similarly to sand bars in a 20-km reach, 125 km downstream from the dam. Areas of sand bars in these two reaches were more similar to each other than to an intervening 11-km reach located 94 km downstream from the dam, both in the pre- and the post-dam eras. Thus, there is not a systematic downstream change in eddy bar size. Local factors such as channel geometry cannot be ignored as major influences causing variability.