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" Abstract L1DAR was flown over Grand Canyon in March and September of 2000 to assess its ability to provide a relatively low-cost. ecosystem-wide change detection tool. Detennining the error associated with the elevation values of the L1DAR points was necessary to ensure that true change in the target resource was being recorded. This paper shall present the methods used to check the vertical accuracy of the LIDAR points. describe the results, and offer conclusions regarding the potential utility of using L1DAR as a change detection tool for monitoring downstream resources that are affected by hydropower operations. Introduction The Grand Canyon Monitoring and Research Center (GCMRC) of/he U.S. Geological Survey studies the effects of water release from Glen Canyon Dam on the ecology of the Colorado River through Grand Canyon (from Lake Powell to Lake Mead) in order to develop release protocols that minimize any adverse effects on the river's ecology. One way that scientists at the GCMRC monitor downstream resources is by measuring changes in sand bar size and volume that are the result of dam operations. Currently. sand bar area and volume changes are repeatedly measured using conventional ground survey techniques. This measurement method can be time-consuming. costly and intrusive, which may limit the change detection to a small percentage of the overall ecosystem. Light Detecting and Ranging (L1DAR) technology seemed to offer a potential way to evaluate sediment changes throughout the ecosystem in a relatively short amount of time in a cost-effective and unintrusive manner. Before any change detection could be preformed using L1DAR, the vertical error of the L1DAR data needed to be assessed to ensure that L1DAR measurements were reflecting true changes in the resource and not simply vertical errors between different L1DAR flights. L1DAR data were acquired over the eight study reaches in late March. in late August, and in middle September 2000 in order to assess changes in sandbar volume due to a spike water release by the Glen Canyon Dam in early September 2000. Topographic data were obtained simultaneously from ground surveys over the same river reaches. This paper assesses the vertical accuracy and precision of the LIDAR points to determine if L1DAR technology provides an adequate tool for monitoring changes in downstream resources. Study Areas The eight areas used in our evaluations are located within the upper one-third of the Colorado River in Arizona (Figure I). Five of these study areas are long-term monitoring sites for sediment storage (RM 2.8, RM 30, RM 43. RM 59, and RM 62.7: Figure I). [RM refers to river mile from Lee's Ferry. Arizona, which is designated as RM 0.] These sites were topographically mapped by ground survey crews during the L1DAR overflights in August and September 2000 and are referred to as the sandbar mapping surveys. In addition. spot elevation surveys of randomly selected L1DAR points were conducted in all eight study areas subsequent to the overflights; these ground surveys are referred to as stakeout surveys. The five long-term monitoring sites are typically surveyed on an annual basis. With respect to sediment monitoring, a change in ground elevation at any location on a sand bar is deemed significant when the change exceeds 25 cm (either positive or negative: Schmidt et aI., 1999). Thus. vertical (elevarion) accuracies to identify such a change need to be at least 25 cm. preferably near tbe 15 cm level. Land survey methods currently employed for sediment monitoring produce vertical accuracies near 3-5 cm. 2