Riffe Lake

The Washington Department of Ecology held a public comment period for the Riffe Lake/Kosmos Flats site between May 25 and June 23, 2020.  One comment was received.  The responsiveness summary can be found at the following link:

Riffe Lake/Kosmos Flats Responsiveness Summary

The Agreed Order became effective on August 19, 2020. Tacoma Power will prepare a remedial investigation, feasibility study, and a preliminary cleanup action plan.  These documents will be available for public comment in the future.

A fact sheet has been prepared about this site and can be viewed HERE.

The town of Kosmos was flooded with water in 1968 to create Riffe Lake, a part of Tacoma Power’s Cowlitz River Project.  Residents were relocated and business including a lumber mill were abandoned and demolished.  Currently, the surrounding land and water are used for recreational activities such as boating, camping and a hang-gliding landing area.

 In 1992, Tacoma Power removed two underground storage tanks (USTs) related to the lumber mill. One of the tanks located near the former mill foundation contained petroleum product and halogenated hydrocarbons.  Petroleum product contaminated surrounding soil at levels exceeding state cleanup standards.  A total of 87 cubic yards of contaminated soil was removed and disposed of appropriately. 


In April 2019, the Department of Ecology (Ecology) was notified about seeps of Bunker C oil found on the eastern bank of Rainey Creek near Champion Haul Road and at the former lumber mill area. In December 2019 as an emergency cleanup action, Tacoma Power capped the seeps to keep oil from moving into Rainey Creek before the water level in Riffe Lake inundated the site. They collected soil and groundwater samples from test pits and soil borings and found other contamination that include diesel, gasoline, and heavy oil. Approximately 11,000 tons of impacted soil were removed and disposed of appropriately.

Possible sources for the seeps are from former lumber mill operations, including old underground storage tanks and piping, or petroleum-impacted soil. 

Contact with these hazardous substances can be harmful to human health and the environment.  Tacoma Power has closed the area to protect the public until cleanup is complete.