New Transformers Coming to Ross Powerhouse

For the first time in over 60 years, Seattle City Light is replacing several large transformers in the Ross Powerhouse at Diablo Lake. Colonial Creek Campground users may see some temporary impacts during the summers of 2016 and 2017.

The transformers in question have the important job of converting the electricity produced from the hydro-generators to a higher voltage before it travels over transmission lines. The existing transformers have been in continuous use since the beginning of Ross Powerhouse’s operations in the 1950s, and they have reached the end of their useful life.

Three transformers can be seen in this photo from 2004

Ross Powerhouse is located on the Skagit River at the upper end of Diablo Lake and it is not accessible by road; all equipment must be transported up Diablo Lake by barge. The transformers are very heavy–up to 80 tons–and the delivery trucks are too long to drive across Diablo Dam to reach City Light’s existing barge loading area. So, City Light built a temporary gravel road near Colonial Creek campground to reach the lake. 

Seattle City Light will deliver seven new transformers and remove six existing transformers over the next two years. Half of the work will be performed during the summer of 2016, with the other half to be completed in summer 2017.


  • The first batch of new transformers will barge to Ross Powerhouse in the second half of June. The process will take approximately ten days.
  • The first batch of old transformers will barge out of Ross Powerhouse in late July 2016 and the process will take about one week

Impacts to Parks users:

  • Most of the parking spaces in the lot immediately next to Highway 20 will be unavailable from approximately June 15 to June 25, 2016, and again in late July, 2016.
  • Signs restricting parking will be posted several days in advance.
  • If towing is necessary, cars will be relocated to the parking lot across Highway 20 near the boat launch.
  • There may be construction noise during the night as transformers arrive by truck and are stored near the access road. The Washington Department of Transportation restricts the hours that very heavy loads can operate on highways.
  • There will be construction noise during the day as transformers are loaded onto special moving vehicles and placed on the barge. Work may begin as early as 6 AM.  The hours of operation are restricted by water levels on the lake and weather conditions.
  • During the transformer deliveries, Diablo Lake will be at normal to high elevations.

If you have any questions, please contact Chris Woelfel, Seattle City Light Project Manager, at 206-684-3553 or

Barge Landing on Diablo Lake Reaches Milestone

A construction crew installs a pre-cast concrete section for the heavy barge landing at Diablo Lake.

Seattle City Light’s contractor for a heavy barge landing dock on Diablo Lake at the utility’s Skagit Hydroelectric Project has reached an important milestone.

Last week, the contractor set pre-cast concrete panels for the barge landing, which is one of the biggest components of the job. Remaining tasks include tightening the panels together, patching any openings, adding handrails and bumpers, and final road grading.

This $1.5 million project is one of the final phases of work needed to replace facilities destroyed by a 2010 rock slide.

City Light uses barges to carry heavy equipment across Diablo Lake to and from its Ross Dam and Powerhouse. The project must be completed to allow for the delivery of six new transformers to Ross Powerhouse scheduled for 2016.

Diablo Lake was lowered about 10 feet for some of the earlier work. It is now back in its regular operating range.

Seattle City Light is the 10th largest public electric utility in the United States. It has some of the lowest cost customer rates of any urban utility, providing reliable, renewable and environmentally responsible power to about 750,000 Seattle area residents. City Light has been greenhouse gas neutral since 2005, the first electric utility in the nation to achieve that distinction.

Workers install a pre-cast concrete section of the heavy barge landing.