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2 Theodore S. Me/is el of. 37 113i UTAH 112 t" ARIZONA <f 50 La" Powell tj ~ ~ ~ G" eMyon D.... 'O~'i}o.O GO' , , km ~i., ., 361 \ , Gaging station ~ ~ " b O;ai'i'P<::- l Fig. 1 Map of the Colorado River downstream from Glen Canyon Dam. environmental management. One major objective of the monitoring programme is to identifY key periods when the ecosystem's sand supply is sufficiently enriched for artificial tloods to rebuild sand bars. Typically, this is done by estimating the system- wide sand mass balance between influx from tributaries and export from Grand Canyon, Daily measurements of suspended transport using isokinetic samplers from cableways are currently required to estimate monthly-to-seasonal sand flux below the dam. CoIlection of these samples is logistically complicated, costly, and provides limited spatial and temporal resolution with respect to the variability of sand transport. The objective of this paper is to evaluate the potential for using LlSST technology to increase the spatial and temporal resolution of suspended-transport monitoring in the Colorado River. RESULTS AND DISCUSSION Initial point data collected at a fixed-depth, near-shore site were obtained by averaging 16 measurements at 2-min intervals during a 24-h deployment starting at 23:00 h GMT on 19 July, 2001. These data were collected using a LlSST-IOO 'Type-B" sensor (Laser In-S.itu ~cattering and Iransmissometry) manufactured by 'Sequoia Scientific, Inc, in Bellevue, Washington, USA, The Type-B is a laser-diffraction based sensor designed to detect suspended particles over a size range of 1.3-250 !!m. An additional description of this technology is reported by Agrawal & Pottsmith (200 I). The LlSST- 100B used in July 2001, was previously evaluated under laboratory and field conditions and its performance is reported by Gartner et at. (200 I). The 720 LlSST point measurements collected at the Grand Canyon gauge in July 200 I, compare well with cross-sectional integrated suspended-sand and silt & clay data collected at a cableway near the test site using a D-77 isokinetic bag sampler (Fig. 2). During the July 2001 test, fluctuating releases from Glen Canyon Dam ranged from about 320-480 m3 s" (typical diurnal pattern of discharge related to hydropower generation at the dam). In addition to accurately tracking the sand concentration, the LlSST-IOOB also recorded the physically-expected increase in sand-concentration variance as flow increased, with peak values ranging from 50 to 140 mg 1" (Fig. 2(a)), Concentrations of silt and clay obtained by the LISST - I OOB were a factor of nine less 02362