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Diablo Dam Road

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Diablo Dam Road is a reservoir road located in Whatcom County, Washington.  Said roadway connects Washington State Route 20 (the North Cascades Highway) to the North Cascades Environmental Learning Center via Diablo Dam.  The roadway atop Diablo Dam was an original part of the structure which had been completed as part of the Skagit River Hydroelectric Project in 1930.  Originally access road access to Diablo Dam was through a winding switchback road located immediately east of the Seattle City Light company town of Diablo.  The modern access road to the southern end of Diablo Dam was built in 1960 during the construction of the North Cascades Highway (then Primary State Highway 17). 




Part 1; the history of Diablo Dam Road

Diablo Dam is one of three structures part of the Skagit River Hydroelectric Project.  The Skagit River Hydroelectric Project is owned and operated by Seattle City Light.  The purpose of the Gorge Dam, Diablo Dam and Ross Dam is to provide electricity to the Seattle.

The permit for the Skagit River Hydroelectric Project was pursued by Seattle superintendent of lighting James Delmage Ross.  The Department of Agriculture approved the project in 1917, and the Seattle city council issued $1,500,000 in bonds to aid in funding construction.  

A 25-mile extension of the Great Northern Railway east out of the Concrete area was constructed to the site of Gorge Dam.  The line largely followed the Skagit River but required a two-mile tunnel be drilled from the powerhouse location at the Seattle City Light company town of Newhalem to the Gorge Dam site.  Construction of Gorge Dam began in 1921 and would be completed in 1924.  Then President Calvin Coolidge would dedicate the Gorge Dam powerhouse on September 17, 1924.  

The then future Ruby Dam (now Ross Dam) site can be seen at the end of the western segment of the Methow Valley Highway (Secondary State Highway 23) on the 1925 Rand McNally Map of Washington and Oregon.  The highway is shown being aligned north of existing Gorge Lake and site of Diablo Lake.  Secondary State Highway 23 appears to be a very similar concept to the modern North Cascades Highway with the major exception being its planned route followed the Methow River to the north of Washington Pass towards Mazama.  


Construction of the arch concrete Diablo Dam upstream from Gorge Dam began in 1927.  The then new dam site required the commissioning of a second Seattle City Light company town known as Diablo.  The structure of Diablo Dam would be complete by 1930, but it would not begin to generate electricity until 1936. 

From the outset Diablo Dam had a roadway affixed to the top of the structure.  The dam was originally connected to the town site of Diablo via a switchback road above the community.  Diablo Dam can be seen below as it was in 1935 (Seattle.gov photo).  Ruby Peak can be seen in the background. 


The 1931 Clason's Road Map of Washington State shows the concept of the Methow Valley Highway was to cross the Cascade Range via the 6,197-foot-high Harts Pass.

During 1932 State Highway Engineer Ike Munson surveyed a new route across the northern Cascade Range.  Munson originally surveyed a route which would pass through Stehekin eastward towards Twisp.  Ultimately a new route was chosen which was similar to the Methow Valley Highway.  The surveyed highway would begin in Marblemount ascending eastward towards Rainy Pass and Washington Pass towards Mazama.  The new route had the advantage in that it could incorporate the already constructed road from Marblemount to Diablo company town. 

The 1939 Rand McNally Map of Washington State shows the future corridor of the North Cascades Highway assigned to several different route numbers.  The highway from Sedro-Woolley to Marblemount is shown to be assigned as Secondary State Highway 17A whereas the Methow Valley Highway is shown as part of Primary State Highway 16.  The Cascade Wagon Road is displayed as Primary State Highway 17.  


During 1953 the North Cross-State Highway Association formed and began to advocate for a northern Trans-Cascade Highway to be completed.  The conceptualized North Cascades Highway is shown following the surveyed 1932 Munson Route on the 1956 Shell Highway Map of Washington.  Primary State Highway 17 is shown to be realigned off of the Cascade Wagon Road from Marblemount towards Newhalem.  From Newhalem a new roadway is shown to be under construction east to Diablo Dam.  


The new alignment of the North Cascades Highway bypassed the Diablo company town via a bridge constructed over Gorge Lake in 1960.  The highway connected to the southern side of Diablo Dam via an extension of the already existing road atop the structure.  

Construction of the remaining North Cascades Highway began in 1959 from Diablo Dam 5.9-miles to Thunder Arm.  During 1964 the North Cascades Highway was reassigned as part of Washington State Route 20 as part of the wider Washington State Highway Renumbering.  On October 2, 1968, Congress had authorized the creation of North Cascades National Park, Ross Lake National Recreation Area and Lake Chelan National Recreation Area.  Despite the newly designated recreational areas the grading of an unpaved North Cascades Highway was completed during 1968.  The unpaved North Cascades Highway (originally known as the North Cross-State Highway) was dedicated on September 29, 1968, amid a crowd of off-highway/high clearance vehicles. 

Below then Washington State Governor Evans can be seen at Rainy Pass during the opening ceremony of the unpaved North Cross-State Highway in a Washington State Achieves sourced photo.  

During 1971 the North Cross-State Highway Association renamed as the North Cascades Highway Association.  Surfacing of the North Cascades Highway was completed during 1972 the highway was rededicated on September 2, 1972.  Below Governor Dan Evans can be seen at the North Cascades Highway dedication ceremony held in Winthrop (sourced Washington State Department of Transportation).  


Part 2; a drive on Diablo Dam Road

The modern alignment of Washington State Route 20 bypasses the company town of Diablo via the bridge over Gorge Lake mentioned in Part 1.  The original roadway to Diablo Dam was aligned through Diablo via a series of switchbacks.  


Washington State Route 20 eastbound from Diablo crosses over Gorge Lake and ascends to the start of Diablo Dam Road.  




Traffic entering eastbound Diablo Dam Road is warned of the narrow single-lane conditions ahead.  


Diablo Dam Road descends to the namesake dam.  The roadway atop the structure is also open to pedestrians and signed with a 10 MPH speed limit.  







Diablo Dam Road crosses over Diablo Dam and makes a right-hand turn at the northern end of the structure.  Diablo Dam is 389 feet high and 1,180 feet long.






Diablo Dam Road runs along the northern end of Diablo Lake.  The road upon crossing Diablo Dam passes the dock for Skagit Tours.  








Diablo Dam Road terminates at the North Cascades Environmental Learning Center which has beach access to Diablo Lake. 





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