Graduate Thesis Or Dissertation
 

Forest harvesting impacts on coarse woody debris and channel form in central Oregon streams

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/c247dw46z

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  • Several streams in the mixed conifer cover type of the Ochoco and Blue Mountains of central Oregon were studied to determine what effects selective harvesting of the riparian timber stand had on the amounts and distribution of coarse woody debris (CWD) in the stream. Pristine streams were measured to develop a data base for the streams of the region. Watersheds that had experienced harvests from 12 to 60 years ago which had from 22 to 93 percent of the basal area removed were then measured to determine the effects of selective harvesting. CWD volumes were computed for all pieces greater than 10 centimeters in diameter at the small end, and at least 1 meter in length. Each piece was classified according to source, association with other debris, age, species, and overall characteristics. No statistical difference could be found in the total volume of CWD when comparing the managed to the pristine streams. Debris volume was found to be directly related to riparian stand basal area, with species composition of the stand being a strong indicator of stand basal area and stream debris volumes. With the exception of large multi-tier jams, which were not present in the six managed streams, the number and distribution of debris associations were found not to be affected by harvesting. Channel morphology measurements were taken along the same 300 meter reaches and included average active channel width and depth, bank slopes, gradient, spherical canopy density, and angular canopy density. Individual channel parameters measured included pool surface area and depth, riffle surface area and depth, depth and length of bank undercutting, and surface area and depth of multiple channel sections. All measurements were based on active channel flow conditions. No significant differences were found between the harvested and pristine streams with regard to active channel width, depth, or streambank slope. Individual pools were larger in volume in the managed streams. Since the number of pools and their average depth was the same as in the pristine streams, the managed streams had a larger volume of pool habitat. Multiple channel units were seven times more prevalent in harvested reaches than in pristine reaches. There were no differences found in the role of CWD in the formation of stream channel units between the types of stream.
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