Cyclic Erosional Instability of Sandbars along the Colorado River, Grand Canyon, Arizona Leland R. Dexter* and Brian L. Cluer** *Department of Geography, Northern Arizona University **Water Resources Division, National Park Service The study and management of regulated rivers have become important issues. A prime example is Glen Canyon Dam and its operational impacts on the downstream environment in Grand Canyon National Park, Arizona. We present an overview of the Glen Canyon Dam environmental issue, a novel methodology for monitoring dam impacts on alluvial sediments, and three years of study relating to the stability of alluvial deposits along the Colorado River. This research uses oblique photographs taken daily, automatically, from twenty-one fixed-position programmable cameras. Digital image-processing techniques created planimetric models of sandbar area from the photos for the period of March 1993 through May 1995. The technique allowed daily tracking of sandbar areas for detection of rapid erosional events. We demonstrated that rapid erosional events occur commonly on Grand Canyon sandbars. Sandbars are unstable over the study period, especially the first two years. Most of the rapid erosional events are associated with weekend or seasonal reduction in flow. Sandbar area frequency is bimodal with negative kurtosis, indicating that measurements taken at long time-steps are not likely to document mean area but rather minima or maxima. Time-series analysis suggests that periods of relative stability occur between rapid area-reducing events. Sandbars appear to adjust in two modes, a short-term adjustment mode occurring over hours and a long-term adjustment mode over days to weeks. The understanding and minimization of rapid-failure events should be increased, and the phenomenon needs to be addressed in any comprehensive sediment management plan. Key Words: sandbar, fluvial erosion, image analysis, Colorado River, Grand Canyon National Park, Glen Canyon Dam. M ay 1996 began a new era in the opera- tion of U.S. dams when Secretary of the Interior Bruce Babbitt signed a record of decision (ROD) concluding an environmental impact assessment process that had spanned more than a decade (U.S. Bureau of Reclamation 1995). The decision specified that Glen Canyon Dam on the Colorado River would be operated under a set of temporary and adaptive flow crite- ria termed the “Interim Flow Prescription” (de- scribed in detail below). The dam would no longer be operated just for power generation, but would also be used to reduce impacts on the downstream riparian environment, with provisions to allow for periodic rejuvenating flood releases. The ROD followed a much-publicized Test Flood released from Glen Canyon Dam between March 26 and April 4, 1996. A major objective of this “engi- neered” flood was to translocate sand from the riverbed onto the adjacent banks in an attempt to rebuild the sandbars in Grand Canyon Na- tional Park, which are valuable resources for plants, fish, birds, and other canyon life. Success of the flood for sandbar regeneration was imme- diately apparent. Nevertheless, the longevity of the elevated sand deposits is still unknown, as is the “optimal” long-term dam-release strategy. In this paper, we present an overview of the Glen Canyon Dam environmental impact assess- ment process, a novel methodology for short-term monitoring of impacts to river sediments, and three years of pre-flood, interim flow results. The data suggest that many Grand Canyon sandbars are prone to very short-term erosional episodes that appear to be closely coupled to dam opera- tion. An “optimal” management strategy will therefore need to accommodate both long-term and short-term attributes of this fluvial system. Annals of the Association of American Geographers, 89(2), 1999, p. 238–266 ©1999 by Association of American Geographers Published by Blackwell Publishers, 350 Main Street, Malden, MA 02148, and 108 Cowley Road, Oxford, OX4 1JF, UK.