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Introduction The closure of Glen Canyon Dam and the begin- ning of flow regulation of the Colorado River through Grand Canyon in 1963 all but eliminated the mainstem sand supply to Grand Canyon and substantially altered the seasonal pattern of flows in the Colorado River. Dam-induced changes in both sand supply and flow have altered the sedimentary processes that create and main- tain sandbars and related habitats, resulting in smaller and coarser grained deposits throughout the ecosystem. From the perspective of river management, the ecological implications associated with such changes are not well understood and are the focus of ongoing integrated science studies. The effects of Glen Canyon Dam operations on fine-sediment resources (i.e., sand and finer material), particularly the erosion and restora- tion of sandbars, are of interest because sandbars are a fundamental element of the Colorado River's geomor- phic framework and the landscape of Grand Canyon (see Webb, 1996; Webb and others, 2002). Sandbars are also of interest in terms of the essential role fine- sediment resources play in other ecosystem processes (U.S. Department of the Interior, 1995). For example, emergent sandbars create terrestrial habitats for ripar- ian vegetation and associated fauna. Similarly, sandbars create areas of stagnant or low-velocity flow that may be used as rearing habitat by the endangered humpback chub (Gila 0'Plza) and other native fish. Recreational river runners and other backcountry visitors frequently use sandbars as campsites. Finally, abundant sand and silt deposits near and above the elevation of typical predam floods contain archeological resources and protect those resources from weathering and erosion. Conservation of Grand Canyon's fine-sediment resources is a primary environmental goal of the Glen Canyon Dam Adaptive .Management Program. Despite this fact, the dam's hydroelectric powerplant operation under the Record of Decision (U.S. Department of the Interior, 1996) continues to erode the limited fine- sediment deposits that exist downstream. Changes in the abundance, distribution, size, and composition of sandbars began to occur under the no action period (his- torical operations) of dam operation from 1963 through 1991. Sandbar erosion continued despite changes in the operation of the dam that resulted from the imple- mentation of the interim operating criteria in 1991 and the modified low fluctuating flow CMLFF) alternative in