<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 />
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<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
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<br />Topping, D. J., Rubin, D. M. & Vierra, L. E., Jr, (2000a) Colorado River sediment transport, I. Natural sediment supply
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