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<br />Erosion and Sediment Transport Measurement in Rivers: Technological and Melhodological Advunces <br />(Proceedings of the Oslo Workshop, June 2002), IAHS Pub!. 283, 2003. <br /> <br />Testing laser-based sensors for continuous in situ <br />monitoring of suspended sediment in the Colorado <br />River, Arizona <br /> <br />THEODORE S. MELIS, DAVID J. TOPPING <br />us Geological Survey, Grand Canyon Monitoring & Research Center, 2255 N. Gemini Dr., <br />Flagstaff, Arizona 8600J, USA <br />tmelis[a:lusl!s.l!OV <br /> <br />DAVID M. RUBIN <br />US Geological Survey, Coastal & Marine Geology, Santa Cruz, California 95064, USA <br /> <br />Abstract High-resolution monitoring of sand mass balance in the Colorado <br />River below Glen Canyon Dam, Arizona is needed for environmental <br />management. In Grand Canyon, frequent collection of suspended-sediment <br />samples from cableways is logistically complicated, costly and provides <br />limited spatial and temporal resolution. In-situ laser sensors were tested in the <br />Colorado River as an alternative method for monitoring the river's suspended <br />transport. LISST data were collected at a fIxed-depth, near-shore site while <br />isokinetic measurements were simultaneously made from a nearby cableway. <br />Diurnal variations in LISST grain size and concentration data compared well <br />with depth-integrated, cross-section data. The LISST was also successfully <br />used to electronically trigger an ISCO 6712 pump sampler to provide <br />continuous monitoring during periods when suspended concentrations <br />exceeded the LISST's measurement range. Initial results indicate that the <br />LISST can provide useful high-resolution suspended-sediment data within the <br />Colorado River, when optics are maintained on a weekly basis. <br /> <br />[suspended-sediment transport, LISST, Colorado River, environmental monitoring) <br /> <br />INTRODUCTION <br /> <br />Closure of Glen Canyon Darn in 1963 (Fig. I) resulted in an ilfunediate and drastic <br />reduction of the Colorado River ecosystem's fme-sediment supply, as well as <br />substantial changes in the river's seasonal transport behaviour (Topping el ai" 2000a, <br />2000b). Today, the annual fine-sediment supply below the dam is about 10% of the <br />pre-dam supply and is delivered to the ecosystem mostly by the Paria and Little <br />Colorado Rivers (Fig. I). Downstream sand inputs are fine (median size ~115 J.!m), <br />while the dam's operation has increased the daily median flow of the Colorado River <br />through Grand Canyon by about a factor or two. Together, these factors cause erosion <br />of existing sand bars and force rapid export of new sand inputs under most current dam <br />operations (Rubin et ai" 2002). The sand bars represent the channel-storage term of the <br />ecosystem's fme-sediment mass balance and restoration of eroded sand bars is a <br />primary objective of an ongoing adaptive environmental assessment and management <br />programme. <br />Owing to the ecosystem's sand-supply limited condition, intensive monitoring of <br />fine sediment below Glen Canyon Dam is a critical requirement for successful <br /> <br />\)~'jb'3 <br />