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<br />Testing laser-based sensors for continuous 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 LISST 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 />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, LISST can <br />support continuous suspended-sediment monitoring in the Colorado River. However, <br />because both the LISST 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 LISST 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 />IAny use of trade, product, or firm names is for descriptive purpose~9nly and does <br />not constitute endorsement by the US Geological Survey. <br /> <br />REFERENCES <br /> <br />Agrawal, Y. & Pottsmith, C. (2001) Laser sensors for monitoring sediments: capabilities and limitations, a survey. (Proc. <br />Seventh Fed. Inter. Sed. Conf. (111)),144-151. <br />Gartner, J, W., Cheng, R. T., Wang, P. & Richter, K. (2001) Laboratory and field evaluations of the LISST-IOO instrument <br />for suspended particle size determinations. Marine GeoJ. 175, 199-219, <br />Rubin, D. M., Topping, D. J" Schmidt, J. c., Hazel, J., Kapllnski, M. & Melis, T, S. (2002) Recent sediment studies refute <br />Glen Canyon Dam hypothesis. Eos 83(25), 273-278, <br />Rubin, D. M. & Topping, D, 1. (2001) Quantitying the relative importance of flow regulation and grain size regulation of <br />suspended sediment transport (n) and tracking changes in grain size of bed sediment (13). Water Resour. Res. 37(1), <br />133-146. <br />Topping, D. J., Rubin, D. M. & Vierra, L. E., Jr, (2000a) Colorado River sediment transport, I. Natural sediment supply <br />limitation and the influence of Glen Canyon Dam. Water Resour. Res. 36(2), 515-542. <br />Topping, D. J., Rubin, D. M., Nelson, J. M., Kinsel, P. J. III & Corson, I. C. (200Gb) Colorado River sediment transport, 2. <br />Systematic bed--elevation and grain-size effects of sand supply limitation. Water Resour. Res. 36(2), 543-570, <br /> <br />02357 <br />