Abstract Title

Session S-06F: Elwah River Restoration: Evolution of Habitats and Ecosystems During a Dam Removal Project

Keywords

Restoration

Start Date

1-5-2014 1:30 PM

End Date

1-5-2014 3:00 PM

Description

Over 6 million tonnes of suspended sediment flowed down the lower Elwha River, Washington, in the first 24 months of a multi-year dam-decommissioning project that started in September 2011. The project strategy involves progressive, simultaneous removal of two dams through a process of controlled drawdowns occurring over a 2-3 year period. For nearly 100 years, the 32-m Elwha Dam, located on the Elwha River 8 km upstream from the river mouth at the Strait of Juan de Fuca, has blocked access of anadromous salmonids to the upper watershed. The construction in 1927 of the upper dam (64-m Glines Canyon Dam located 22 km upstream from the river mouth) further restricted the river. In all, about 34 million tonnes of sediment were trapped behind both dams. The dam-removal process is expected to be completed in 2014, but during and after removal, sediment will be allowed to transport downstream. Turbidity in the lower river, measured continuously, was elevated during the two winter flood seasons of the project with a median value of 140 FNU and 560 FNU in years one and two of the project, respectively. For comparison, median turbidity in the river upstream from the project, measured over year one, was 2 FNU. Turbidity values more than 200 FNU were common when discharge exceeded 100 cubic meters per second, though turbidity rarely exceeded 1,500 FNU in year one. In year two, turbidity approached or exceeded 1,500 FNU for one or more days on five occasions. A relation between suspended-sediment concentration and turbidity (developed from the suspended-sediment sampling near the turbidity sensor) allowed the calculation of suspended-sediment load at 15-minute intervals. Samples from the river showed that suspended-sediment concentration was generally less than 1,000 mg/L in year one but commonly exceeded 1,000 mg/L in year two as sediment from upstream of Glines Canyon Dam was released. From May to July 2012 and from October 2012 to July 2013, a near-continuous load of sediment was carried downstream.

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May 1st, 1:30 PM May 1st, 3:00 PM

Suspended-sediment load in the lower Elwha River, Washington, during dam decommissioning

Room 602-603

Over 6 million tonnes of suspended sediment flowed down the lower Elwha River, Washington, in the first 24 months of a multi-year dam-decommissioning project that started in September 2011. The project strategy involves progressive, simultaneous removal of two dams through a process of controlled drawdowns occurring over a 2-3 year period. For nearly 100 years, the 32-m Elwha Dam, located on the Elwha River 8 km upstream from the river mouth at the Strait of Juan de Fuca, has blocked access of anadromous salmonids to the upper watershed. The construction in 1927 of the upper dam (64-m Glines Canyon Dam located 22 km upstream from the river mouth) further restricted the river. In all, about 34 million tonnes of sediment were trapped behind both dams. The dam-removal process is expected to be completed in 2014, but during and after removal, sediment will be allowed to transport downstream. Turbidity in the lower river, measured continuously, was elevated during the two winter flood seasons of the project with a median value of 140 FNU and 560 FNU in years one and two of the project, respectively. For comparison, median turbidity in the river upstream from the project, measured over year one, was 2 FNU. Turbidity values more than 200 FNU were common when discharge exceeded 100 cubic meters per second, though turbidity rarely exceeded 1,500 FNU in year one. In year two, turbidity approached or exceeded 1,500 FNU for one or more days on five occasions. A relation between suspended-sediment concentration and turbidity (developed from the suspended-sediment sampling near the turbidity sensor) allowed the calculation of suspended-sediment load at 15-minute intervals. Samples from the river showed that suspended-sediment concentration was generally less than 1,000 mg/L in year one but commonly exceeded 1,000 mg/L in year two as sediment from upstream of Glines Canyon Dam was released. From May to July 2012 and from October 2012 to July 2013, a near-continuous load of sediment was carried downstream.