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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5164

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Summary and Conclusions

Cougar Reservoir was a substantial source of sediment to the South Fork and mainstem McKenzie Rivers between spring 2002 and December 2003. However, sediment transport decreased during calendar year 2004, effectively muting the relative importance of Cougar Reservoir and elevating the relative contribution from upstream sources on the mainstem. Evidence for the importance of Cougar Reservoir releases includes the increased turbidity and suspended-sediment concentrations at CGRO and VIDA as compared to upstream sites not influenced by Cougar Reservoir, the large proportion of the annual load at VIDA that was accounted for by the load at CGRO during water years 2003 and 2004, and the increased percentage of fine material collected by infiltration bags during 2003–04. Erosion of the deltaic sediments within the drawn-down, upstream reaches of the reservoir, which occurred primarily during spring 2002 and then episodically during the winters of 2002 and 2003, was apparently the largest source of sediment; however, the lack of turbidity monitoring at VIDA, MRBO, and BLUE until January 2003 make complete assessment of the relative contributions from the reservoir impossible. Judging from a mass balance on the suspended-sediment loads, most of the suspended sediment transported from Cougar Reservoir to the McKenzie River during 2003–05 also was transported downstream beyond the VIDA station.

DDT and its metabolites (ΣDDx) were detected in the reservoir and streambank sediments, as well as in upland soils, prior to this study. However, the concentrations were low, requiring a large re-mobilization of sediments, with suspended-sediment concentrations possibly greater than 100 mg/L, for the ΣDDx to be transported downstream in detectable concentrations. The reservoir likely was a sink for DDT applied to the South Fork basin during the 1960s, whereas applications to the other sections of the McKenzie River basin probably were acting as sources since the 1960s. Transport of sediment from Cougar Reservoir during the drawdown may have caused small amounts of ΣDDx to be released from the reservoir but ΣDDx probably would have been diluted to non-detectable concentrations by the time it entered the McKenzie River. The detection of low concentrations of ΣDDx only in infiltration bags downstream of Cougar Reservoir and at VIDA supports the idea that the effect of the ΣDDx release from the reservoir was limited.

Lessons learned from the experience at Cougar Reservoir can be useful for reservoir management within the Willamette River Basin and for future reservoir construction if substantial drawdowns below the normal low pool are required. Although careful management of reservoir pool elevations and streamflows might avoid some problems from migration of exposed channels and downstream sediment transport, some amount of erosion may be unavoidable. Installation of a complete network of turbidity monitoring, coupled with collection of suspended-sediment data prior to the drawdown, would allow for better post-construction evaluation of the role of the construction on sediment transport and areas of likely deposition. The role of normal flood-control reservoir operation practices in downstream deposition of fine materials also remains unclear. Additional investigations of sediment deposition patterns, using either freeze cores, infiltration bags, or other methods, may help confirm whether processes common to reservoir management contribute to increased deposition of fine material. The generation of periodic, bed-moving events in reservoir tailraces also may help minimize the accumulation of fine sediment.

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