Another Look at Mossyrock Dam


On Tuesday, The Chronicle toured the Mossyrock Dam with a hydro mechanic and staff from from Tacoma Public Utilities, the owner of the dam and beneficiary of the power it produces.

To read more about that, click here or see the Thursday, June 23, edition of The Chronicle.

As it happened, even a 1,300 word-story could not summarize all the worthwhile facts about the dam, its design and construction. So, here are some followup factoids about the dam, which is the tallest in Washington at 606 feet from bedrock to the top.


About the Dam

As spokesperson Monika Sundbaum put it, “when the (Mossyrock) dam was completed in 1968, it was a truly impressive feat of engineering and construction.”

The contractor set a new world record for concrete placement: 102,000 cubic yards in one month. The project required the placing of more than 1.25 million cubic yards of concrete in total. For context, a single cubic yard of concrete weighs 4,050 pounds.

Sundbaum also said contractors built “the fastest cableway ever used in dam construction” to move all that concrete.

On May 16, 1969, over 1,200 people gathered for a three-day long dedication of the Mossyrock Dam. Speakers included Gov. Dan Evans and United States Sen. Henry M. Jackson. Tours and demonstrations were provided on the following days.


On Tacoma Power

Tacoma Power — which is just one of the Tacoma Public Utilities along with Tacoma Water and Tacoma Rail and was formerly called Tacoma City Light — is a cost-of-service organization that employs around 800 people. As the utility both creates and distributes electricity, some of those jobs are folks who spend their days watching the power market, calculating earnings and costs of production versus output.

The Mossyrock Dam is on Bonneville Power’s grid, meaning that it is one of many contributors to electricity used by states throughout the Northwest.

Tacoma Power operates four hydroelectricity projects, three of which are made up of more than one dam. The Cowlitz River Project includes the Mossyrock and Mayfield dams. The Nisqually River Project includes the LaGrande Dam, which was first built in 1912, and the Alder Dam, built in 1945. Though it is owned by the City of Aberdeen, Tacoma Power also operates the powerhouse at the Wynoochee Dam. Finally, the utility’s Cushman Hydro Project includes Cushman Dams No. 1 and No. 2 in Mason County.

Through those four projects, Tacoma Power generates about 45% of the electricity it sells to customers, according to Sundbaum. The other 55% is purchased from Bonneville Power.

“We contribute approximately $1.6 million to Lewis County and approximately $440,000 to the local school districts each year,” Sundbaum said.