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Updated flood frequencies and a canal breach on the upper Klamath River

Abstract

During the current water year, the upper Klamath River basin has experienced higher than normal winter and spring flows. In addition, a landslide breached a diversion canal downstream of the J.C. Boyle dam and caused secondary erosion and sedimentation in the “bypass” reach of the Klamath River. The peak flows and landslide may have influenced fish habitat and river geomorphology.

I updated existing flood frequency analyses for four gauges in the upper Klamath River basin using new annual peak streamflow data. I determined that the new flood frequencies reduce the return interval for bed mobility threshold flows at three sites, and increase the return interval of flows over the mobility threshold at two sites, suggesting that existing interpretations about sediment mobility and disruption of fish habitat in parts of the upper Klamath River basin may need to be refined. I also identified differences in flood frequency estimates based on the method used to analyze annual peak streamflow data.

I evaluated the effects of the December 2005 landslide that breached the canal feeding water to the JC Boyle powerplant. The landslide deposited sediment in the Klamath River and the subsequent closure of the canal resulted in increased flows in the river. I expect the effects of the canal breach on downstream fish habitats to be minor because of the short duration of the canal closure and the high flows in the river since January 2006 that likely mobilized the impinging sediment.

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