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The grain size gap and abrupt gravel-sand transitions in rivers due to suspension fallout

Lamb, Michael P. and Venditti, Jeremy G. (2016) The grain size gap and abrupt gravel-sand transitions in rivers due to suspension fallout. Geophysical Research Letters, 43 (8). pp. 3777-3785. ISSN 0094-8276.

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Median grain sizes on riverbeds range from boulders in uplands to silt in lowlands; however, rivers with ~1–5 mm diameter bed sediment are rare. This grain size gap also marks an abrupt transition between gravel- and sand-bedded reaches that is unlike any other part of the fluvial network. Abrupt gravel-sand transitions have been attributed to rapid breakdown or rapid transport of fine gravel, or a bimodal sediment supply, but supporting evidence is lacking. Here we demonstrate that rivers dramatically lose the ability to transport sand as wash load where bed shear velocity drops below ~0.1 m/s, forcing an abrupt transition in bed-material grain size. Using thresholds for wash load and initial motion, we show that the gap emerges only for median bed-material grain sizes of ~1–5 mm due to Reynolds number dependencies in suspension transport. The grain size gap, therefore, is sensitive to material properties and gravity, with coarser gaps predicted on Mars and Titan.

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Lamb, Michael P.0000-0002-5701-0504
Additional Information:© 2016 American Geophysical Union. Received 20 JAN 2016; Accepted 8 APR 2016; Accepted article online 13 APR 2016; Published online 29 APR 2016. Ideas for this paper developed thanks to discussions with M. Church and R. Ferguson. We thank M. Church for a helpful informal review, and R. Ferguson and an anonymous reviewer for formal reviews. Nicola Rammell (SFU) compiled the Alberta rivers data. M.P.L. acknowledges support from the US National Science Foundation (EAR-1233685). J.G. V. acknowledges support from a NSERC Discovery Grant.
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Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
Issue or Number:8
Record Number:CaltechAUTHORS:20160715-085241851
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Official Citation:Lamb, M. P., and J. G. Venditti (2016), The grain size gap and abrupt gravel-sand transitions in rivers due to suspension fallout, Geophys. Res. Lett., 43, 3777–3785, doi:10.1002/2016GL068713
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:69048
Deposited By: Tony Diaz
Deposited On:27 Jul 2016 21:03
Last Modified:03 Oct 2019 10:18

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