Seattle City Light Mill Pond Dam Removal

In 2013 EcoAnalysts, Inc. was contracted by Seattle City Light (SCL) to design and implement a benthic macroinvertebrate monitoring program for the Boundary Hydroelectric Project as part of the license implementation phase. EcoAnalysts worked with the Boundary project biologists to select sites throughout tributaries to the Pend O’Reille River. Macroinvertebrate communities are being measured to monitor overall water/habitat quality in the project area, as well as from a fish food availability perspective. During the site selection process we saw an opportunity to evaluate macroinvertebrate community response to removal of Mill Pond Dam on Sullivan Creek, for which we selected a Before-After/Control-Impact (BACI) sampling design.

Mill Pond Dam was constructed in 1909 by the Inland Portand Cement Company for hydroelectric purposes. The dam was located on Sullivan Creek, was 134 feet long by 55 feet tall, and was removed in summer 2018. The purpose of the project was to remove the Mill Pond Dam and restore the natural riverine conditions along Sullivan Creek. Restoring these riverine conditions will benefit the natural habitat by restoring the downstream transport of coarse sediment and large woody debris, riparian and floodplain environments, and benefits to water quality and reduce water temperatures during the summer warm weather.

During the first year of the project, EcoAnalysts conducted the field sampling, laboratory taxonomic analysis of benthic samples, data analysis and a comprehensive report on benthic macroinvertebrate community condition. During the second year, we trained SCL personnel with the field collection of biological samples, and since then SCL staff or affiliates have conducted the field sampling. EcoAnalysts has processed benthic macroinvertebrate samples each year since. Laboratory analysis of benthic macroinvertebrate samples followed Pacific Northwest (PNW) level 2 taxonomic effort level, usually genus or species. Annual macroinvertebrate community data are being accumulated for a full analysis of dam removal impacts once the post-dam removal data collection period is complete.

Crews removing Mill Pond Dam

Boundary Hydroelectric Project from its Vista House