Response of a stream in disequilibrium to timber harvest |
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Authors: | Burchard H Heede |
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Institution: | (1) USDA Forest Service Rocky Mountain Forest and Range Experiment Station Forestry Sciences Laboratory, Arizona State University Campus, 85287-1304 Tempe, Arizona, USA |
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Abstract: | Timber was harvested on South Fork of Thomas Creek, White Mountains of Arizona, USA, for the first time in 1978–1979. This
caused significant increases in annual flow volumes and annual instantaneous peak flows. North Fork remained untouched, but
both streams were in disequilibrium before harvest time. Due to wetter years during the postharvest period, North Fork also
experienced some flow increases, but the difference was not significant. Flow increases cause increased erosion in disequilibrium
channels. While in South Fork channel cross sections enlarged by 10% since preharvest time, those in North Fork enlarged by
only 2.5%. The number of knickpoints tripled in South Fork, which was about double that in North Fork. Knickpoint development
resulted in destruction of the natural control structures (log steps and transverse gravel bars) in South Fork (47%), while
in North Fork they increased by 23%. Knickpoints are scarps on the channel bed that have the appearance of gully headcuts.
The tripling of the number of knickpoints signifies that adjustment processes of the bed profile are intensified drastically
in South Fork. The geomorphic changes signify that due to increases in discharge, the extent of disequilibrium is exacerbated
in South Fork. Yet, volumes of erosion are relatively small, as will be sediment volumes leaving the watershed at a given
time, because of the stepwise sediment transport occurring in this ephemeral stream. |
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Keywords: | Timber harvest Streamflow Channel erosion Dynamic equilibrium Knickpoint |
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