Journal of Geophysical Research: Earth Surface
© American Geophysical Union
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ISI Journal Citation Reports © Ranking: 2015: 27/184 (Geosciences Multidisciplinary)
Online ISSN: 2169-9011
Associated Title(s): Journal of Geophysical Research
River terraces not a good measure of ancient river behavior
When a river courses along one path for thousands of years, it can smooth the riverbed. As the river erodes the bedrock, the river level drops below the old riverbed. The beveled bedrock layers flanking the new river, made up of the old riverbed, are known as strath terraces. These terraces are commonly assumed to record the profile of the river's paleochannel. In devising a simplified analytical model to calculate the slope of a strath terrace, Finnegan found that the fidelity of strath terraces as recorders of river properties cannot be relied upon under the circumstances common to the formation of many terraces.
In many cases, a river carving its way through its bedrock base does not always do so in a smooth and progressive process. Instead, river erosion can proceed in fits and starts, spurred by tectonic uplift, layering of bedrock material, and changes in sea level that drive changes in river flow rates or riverbed erodibility. In such cases, the changing conditions cause knickpoints in the river—regions where the slope of the riverbed changes sharply—to propagate upstream. The author found that the slope of a strath terrace is determined not by the slope of the historical riverbed, as is commonly thought, but rather by the slope of the river channel upstream of the knickpoint, the propagation rate of the knickpoint, and the riverbed incision rate upstream of the knickpoint. The author suggests that the only condition in which the slope of a strath terrace is representative of the historical river channel is when there is no riverbed incision upstream of the knickpoint.