Heejun Chang

Date of Award

Spring 9-22-2017

Document Type


Degree Name

Master of Science (M.S.) in Environmental Science and Management


Environmental Science and Management

Physical Description

1 online resource (x, 94 pages)


Turbidity -- Oregon -- Clackamas River Watershed, Storms -- Environmental aspects -- Oregon -- Clackamas River Watershed, Water quality -- Oregon -- Clackamas River Watershed




Turbidity is a useful parameter that can be utilized to help understand the water quality in a river and is an expression of the optical properties of a liquid that cause light rays to be scattered and absorbed rather than transmitted in straight lines. A total of 41 storm events occurring during water years 2006-2012 were analyzed for this study. A hysteresis index (HI) was used to assess the difference in turbidity on the rising and falling limbs of a storm-hydrograph. The upstream Carter Bridge site exhibited a clockwise (C) hysteresis in 38 of 41 storm events and counter-clockwise (CC) hysteresis in three storm events. The downstream Oregon City site exhibited clockwise hysteresis in 29 of 41 storm events and counter-clockwise hysteresis in 12 storm events. Paired t-test comparisons of calculated HI measured during storm events showed that the upstream forested site Carter Bridge had a statistically significant higher HI than the downstream Oregon City site, suggesting that particles that contribute to increasing turbidity and suspended sediment at the upstream site are delivered to the river earlier in the storm event in comparison to the downstream Oregon City site. In contrast particulate matter and suspended sediment was more likely to be higher on the receding limb of the storm hydrograph at the downstream site in comparison to the upstream monitoring location.

Multiple linear regression analysis determined the major hydrological and meteorological controls influencing turbidity over the period of a storm event. The log value of the change (Log ΔQ) in discharge explained 81% of the log value of change in turbidity (Log ΔTb) at Carter Bridge and 48% of the change in turbidity at Oregon City for all storms. Log ΔQ explained 85% and 50% variations of Log ΔTb at Carter Bridge and at Oregon City, respectively in the wet season. Log ΔQ explained 82% of Log ΔTb at Carter Bridge during the Dry Season and together with 3-day antecedent precipitation, Log ΔQ explained 84% of variation in Log ΔTb at Oregon City during the Dry Season. The findings of this study, which offers information about the dynamics that lead to increased turbidity events, could be helpful to researchers, regulatory agencies and water resource managers in maintaining high water quality in rivers.

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