Hydrology, Cryosphere & Earth Surface Research Spotlight

Where Does the Bed of a River Change from Gravel to Sand?

Analysis of the Fraser River suggests that riverbed sediments shift abruptly depending on river slope changes and water speed.


Along a riverbed, sediment grains often shrink from gravel sized to sand sized farther downstream. Scientists have documented this phenomenon well, although they are still puzzled by certain details of the gravel-to-sand transition, including why exactly the transition occurs and why it occurs more abruptly in some rivers and more gradually in others.

Venditti and Church present new observations from the Fraser River in British Columbia as a source of potential answers to these questions. The authors analyzed 3 decades’ worth of observations about sediment and hydraulics from the river and found that at about 100 kilometers from the river’s mouth, sediment size abruptly changes from gravel to sand sized. The authors discovered that this shift was associated with a rapid decrease in shear stress on the sediment bed.

The authors also found that the river’s slope decreases significantly at this point, which they propose is an important contributing factor in the gravel-to-sand transition. Although the newest findings agree with previous research, the authors note that further exploration is needed to fill in the gaps in knowledge of this phenomenon. (Journal of Geophysical Research: Earth Surface, doi:10.1002/2014JF003147, 2014)

—JoAnna Wendel, Staff Writer

Citation: Wendel, J. (2014), Where does the bed of a river change from gravel to sand?, Eos Trans. AGU, 95(49), 472, doi:10.1002/2014EO490017.

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