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<br />01961 <br /> <br />Testing laser-based sensors for Conl;nuous in situ monitoring of suspended sediment 7 <br /> <br />recorded about a factor of seven increase in sand concentration and about a 50% <br />decrease in median grain size of sand (13 abruptly decreased). This change in sand- <br />transport occurred in direct response to enrichment of the river's sediment supply <br />following tributary inputs (Figs 3(a) & 4(b)) rather than simply a diurnal change in dis- <br />charge. Results such as these suggest that L1SST data will be suitable for calculating <br />13 at higher spatial and temporal resolutions than those that are presently obtained using <br />cableway sampling methods. A similar monitoring approach may also have utility <br />where high flows are released from dams to accomplish spawning habitat restoration <br />and maintenance through evacuation of sand and silt from gravel-bed interstices. This <br />approach using (13) may also be applied to other sediment transport environments. <br /> <br />j <br /> <br />CONCLUSIONS <br /> <br />Overall, the results of these initial field tests indicate that, with frequent maintenance <br />of optics and when used in combination with automated pumping samplers. L1SST can <br />support continuous suspended-sediment monitoring in the Colorado River. However, <br />because both the L1SST and the pump sampler provide only point data, it is still <br />necessary to obtain depth-integrated, cross-section measurements using isokinetic <br />samplers frequently enough to develop reliable box coefficients. <br /> <br />Acknowledgements The authors wish to thank Francisco Simoes, Christopher Magirl, <br />Andreas Krause and one anonymous reviewer for thoughtful comments and <br />suggestions made during the preparation of this report. We also thank Jeffrey Gartner <br />(US Geological Survey) and David Jay (Oregon Graduate Institute) for generously <br />contributing their time and LISST instruments during these early tests in the Colorado <br />River. Special thanks also go to the following sediment-transport technicians who <br />worked relentless at the Grand Canyon cableway collecting numerous isokinetic <br />suspended-sediment samples used to verifY L1SST measurements: Elizabeth Fuller. <br />Zan Rubin, Erin Todd, David Altezio. Ron Griffiths, Chloe Bonamici, Krissy Kalloy, <br />Mike Cabrera, Tom Sable and Bob Smith. <br />'Any use of trade, product, or firm names is for descriptive purposes only and does <br />not constitute endorsement by the US Geological Survey. . <br /> <br />^ <br /> <br />REFERENCES <br /> <br />Agrawal. Y. & Pottsmilh, C. (2001) Laser sensors for monitoring sedimenlS: capabilities and IimilallOns, a survey. (Proc. <br />Seventh Fed. Inter. Sed. Conr. (111)).144-151. <br /> <br />Ganner, J. W.o Cheng, R. T.. Wang. P. & Richter. K (2001) Laboratory and field evaluatIOns afthe L1SST-IOO Instrument <br />for suspended particle size detenninations. Marine Geol. 175. 199-219. <br /> <br />Rubin, D. M.. Toppmg, D. J., Schmidt, J. c., Hazel, J., Kaplinski, M. & Melis, T. S. (2002) Recent sediment studies refute <br />Glen Canyon Dam hypothesis. Em; 83(25), 273-278. <br /> <br />Rubin, D. M. & Topping. D. J. (2001) Quantifying the relatl'ye importance of now regulation and gram sIze regulation of <br />suspended sediment trnnspon ((1) and tnlcking changes in grain size of bed sediment (p). Water Re.wur Res. 37(1). <br />133-146. <br /> <br />Topping, D. J, Rubin. D. M. & Vierra, L. E., Jr. (2000a) Colorado River sedimenttranspon, I. Natural sediment supply <br />limitation and the influence of Glen Canyon Dam. Water Resour. Res. 36(2), 515-542. <br /> <br />Topping. D. J., Rubm, D. M., Nelson, J. M.. Kinsel. P. J. III & Corson, I. C. (2000b) Colorado RIver sediment transpon. 2. <br />Systematic bed-elevatlon and gram-size efTects of sand supply limitation. Wafer Re.W/Jr. Rt's. 36(2), 543-570. <br />