Municipal water source turbidities following timber harvest and road construction in western Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/w66345665

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  • Many municipalities throughout western Oregon rely upon forested watersheds as a source for domestic water supply. These watersheds are commonly managed by state or federal agencies or private corporations for timber production. Activities related to forest management within municipal watersheds have the potential to adversely affect water quality. Timber harvesting and/or road construction operations may accelerate rates of surface and/or mass soil erosion and subsequent stream sedimentation. This study investigated the effects which timber harvesting and road construction operations had on source water turbidity levels within 13 municipal watersheds throughout western Oregon. Turbidity records were obtained from municipal water treatment facilities and ranged from 5.1 to 16.9 years in length. Information regarding forest land use activities which occurred during 1980-91 was obtained from Oregon Department of Forestry records. An analysis of turbidity residuals before, during and after known timber harvesting and/or road construction operations indicates that forest land use alone did not result in sustained increases in turbidity levels. In some cases, forest operations apparently exacerbated the effects of a severe storm which occurred along the northern Oregon Coast during January of 1990. Mass soil movements occurring in two watersheds as a result of the storm led to large, but short-term increases in turbidity levels. The source of increased turbidities not associated with the 1990 storm could not be identified due to a lack of information regarding sediment sources and transport processes. A second analysis was performed which quantified the level of disturbance resulting from forest operations occurring during each year of record. Mean water year turbidity and the mean of the 10 highest water year turbidities were regressed against the disturbance level for all watersheds. Disturbance levels were based on the type and areal extent of forest operations. Simple linear regression analyses indicate that the relationship between turbidity and disturbance level over the range of data encountered was insignificant (α = 0.05). Impoundments located within the surface water system of many of the study watersheds affected turbidity levels. By acting as a settling basin for sediment particles, impoundments reduced turbidity levels when municipal intakes were located within the structures. Mean turbidity, the standard deviation in turbidity, and the mean of the 10 highest turbidities were all inversely related to impoundment volume. Although the results of this study indicate that forest operations were not a direct casual factor leading to increased source water turbidity levels, any forest management activity has the potential to negatively affect stream water quality. Monitoring efforts designed to investigate the effects of forest land use on water quality should be implemented with a set of predetermined objectives and methods to ensure reliable results.
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