Riparian canopy and channel response to hillslope disturbance in Elk River Basin, southwest Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/f4752j88x

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  • Elk River is a sixth order stream, draining a 200 square kilometer basin in the Klainath Mountain province of southwestern Oregon. Timber harvesting began in the basin during the 1950's, with peak removal of wood occurring in the mid to late 1960's. This activity lead to an increase in the frequency of landsliding and surface erosion. The downstream effects of this increase in sediment were evaluated by interpreting aerial photography spanning the period 1956 to 1979. Opening or enlargement of the riparian canopy cover was used as an indicator of sediment impacts. Results indicate that in most basins, changes in downstream channel conditions could not be directly linked to the increase in sediment delivered from harvested areas. Opening on higher order channels occurred throughout the study period, but did not exhibit patterns indicative of a large pulse of sediment moving through the system. Rather, opening in fourth to fifth order channels was attributed to the interaction between local sediment sources and channel morphology, with unconstrained, flat channel segments exhibiting chronic opening. The distribution of constrained and unconstrained channel reaches was defined by a Channel Morphology Index (CMI), which compared relative width and gradient changes to channel averages. A 100-year event in 1964 did not produce effects in Elk River Basin on the same order of magnitude as other basins in the Pacific Northwest. Downstream effects were important in a few areas where landslides and high rates of surface erosion were directly linked to opening and channel widening downstream. Limited channel response to hillslope. disturbance in Elk River Basin over the period 1956-79 was attributed to four factors: lack of debris flows in most areas of the basin, channels constrained by competent hillslopes and bedrock limiting the extent of opening, channel conditions inherited from storms prior to 1956, and essentially pristine conditions over much of the basin at the time of the 1964 storm. Finally, sequential air photo interpretation proved useful in deciphering disturbance history and analyzing processes which create downstream effects.
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