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Evaluation Methods The accuracies of elevation data provided by L1DAR were evaluated using two approaches. In the first approach we visited randomly selected L1DA R point locations in the tield, used a survey method called "stakeout" to move to the exact horizontal position of the L1DAR point. collected the elevation via conventional survey techniques and recorded the ground cover type, and then compared the elevation values. The advantage of this approach is that we could identify the type of ground surface at each L1DAR point location. General ground surface categories included bare sand, cobbles. boulders. talus, and rock ledges. The survey stakeout procedure was prefonned through an iterative process where a rod man moves to the known L1DAR position being directed by a survey gun operator who occupies a known control point. Control points were surveyed using L I/L2 phase receivers with multiple vectors from Arizona Federal Base Network monuments. The surveyor relays the distance and direction that the rodman needs to move to get to the L1DAR point. When the rodman was within 10 cm of the L1DAR point position. the surveyor recorded the ground surface elevation using conventional survey techniques while the rodman recorded the type of ground surface. Both the ALMS and RAMS data were evaluated al each study area. except in study area RM 59 where only RAMS data were examined. In order to obtain a larger statistical sample and representation of the elevation error at each study site. we employed a second evaluation method that compared L1DAR point elevations with the ground survey point elevations thaI were acquired at the five long-tenn monitoring areas during the L1DAR over flights. Ground survey sandbar mapping was accomplished using Topcon electronic total stations equipped with TDS digital data collectors. Surveyed points were acquired in Arizona State Plane - Arizona Central coordinate system with atmospheric pressure. earth curvature and scale factor accounted for. Accuracy and precision of these techniques have been assessed by Kaplinski el al. [1995]. who also verified all benchmark and back site relationships. Verification of horizontal position and elevation data found that ground points have a horizontal error of <0.1 m and a vertical error <0.05 m. Volume calculations derived from topographic surface models created from replicate daily surveys at one site were shown to vary less than three percent [Beus el al., 1992J. For this comparison. we only used L1DAR points that occurred within a 0.5-m radius of a ground topographic survey point. Ifmore than one L1DAR point occurred within this radius for a particular ground survey point, then a distance-weighted average L1DAR elevation was calculated for that ground survey point. Only points occurring on non-vegetated ground were considered in this particular analysis. The L1DAR versus sandbar mapping ground survey elevation comparisons did not separate bare sand, boulder. talus. and cobble surfaces. Any change detection analysis requires the use ofa measurement technique that provides acceptable levels of reproducibility. as well as acceptable levels of accuracy. We assessed the reproducibility of L1DAR technology using the RAMS data that were acquired at the long-tenn monitoring sites at two separate limes (in lale August and in mid-September of 2000). We performed this analysis at study area RM 59 because the study area has large tracts of bare-ground and vegetated surfaces at elevations high enough to have been unaffected by the intervening (early September) spike-flow release from the Glen Canyon dam. Five separate bare-ground and vegetated-ground surfaces within study area RM 59 were selected for our analysis based on the orthorectified CIR image data of the study. The bare-ground surfaces included sand and rock debris; the vegetated-ground surfaces included mostly dense tamarisk groves and scattered mesquite trees. The shoreward border established for each bare-ground and vegetated-ground surface unit was several meters from the shoreline to minimize topographic change in the areas due to the intervening high flow levels. Within each established bare-ground and vegetated- ground unit at the five test sites, we collected all pre-spike-flow L1DAR elevation points that had at least one post-spike-flow L1DAR elevation point within a radius of 0.5 m. When more than one post-spike- flow L1DAR point occurred within 0.5 m ofa pre-high-flow L1DAR point. a dislance-weighted average elevation was calculated from these post-spike-flow elevation points. 4