What Causes Pools below Waterfalls to Periodically Fill with Sediment?
Deep pools below waterfalls are popular recreational swimming spots, but
sometimes they can be partially or completely filled with sediment. New
research showed how and why pools at the base of waterfalls, known as
plunge pools, go through natural cycles of sediment fill and evacuation.
Beyond impacting your favorite swimming hole, plunge pools also serve
important ecologic and geologic functions. Deep pools are refuges for fish
and other aquatic animals in summer months when water temperatures in
shallow rivers can reach lethal levels. Waterfalls also can liquefy
sediment within the pool, potentially triggering debris flows that can
damage property and threaten lives. Over geologic time, pools are
importance because the energetic waterfall jet can erode the rock walls of
the pool—slowly moving the waterfall upstream, while simultaneously
creating a deep canyon in its wake.
Reporting in the journal Geology today, Joel Scheingross of the
University of Nevada Reno, and Michael Lamb of the California Institute of
Technology, provided a new theoretical framework to predict when plunge
pools fill with sediment, and when they subsequently evacuate that sediment
exposing the rock walls of the pool to erosion. They showed that waterfall
plunge pools tend to fill with sediment during modest river floods when
sediment is transported in the reach upstream of the waterfall, but the
waterfall jet is too weak to move all the sediment it receives from
upstream. In contrast, during large floods, a strong waterfall jet can more
efficiently move sediment up and out of the pool, outpacing the delivery of
sediment from upstream, and exposing bedrock to erosion.
Scheingross and Lamb showed that waterfall plunge pools are most likely to
fill with sediment following landscape disturbances—such as wildfire or
landslides—that cause large influxes of sediment to rivers, or during
prolonged droughts when river floods are rare. This information is useful
to scientists and land managers interested in maintaining habitat and
mitigating natural hazards. They also showed that bedrock erosion in plunge
pools likely occurs only during large, infrequent floods, which tend to
happen every 10 years or even less frequently. Therefore, the slow march of
waterfalls upstream over geologic time likely occurs by fits and starts at
a cadence set by extreme flood events.
Mass balance controls on sediment scour and bedrock erosion in
waterfall plunge pools
Joel Scheingross and Michael Lamb
Contact: jscheingross@unr.edu, University of Nevada Reno, Department of
Geological Sciences and Engineering, Reno, Nevada
URL:
https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G48881.1/598762/Mass-balance-controls-on-sediment-scour-and
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