Annex Creek and Salmon Creek Dam Penstocks
History of the Annex and Salmon Creek Penstocks • est. 1912-1914
The original Annex Creek penstock was a riveted steel pipe 7,097 feet long. It varied in size from 42 inches in diameter and ¼ inch thick at the valve house, to 34 inches in diameter and 5/8 of an inch thick at the powerhouse. The original penstock was installed in 1915 and remained in service for over 100 years until it was replaced in 2024.
The original Upper Salmon Creek penstock is a riveted steel pipe 4,290 feet long. It varies in size from 38 inches in diameter at the valve house, to 30 inches in diameter at the bifurcation to the lower penstock installed in 1984. The original upper penstock was installed in 1914 and remains in service today. A four-year project to replace the existing penstock began in 2026.
The original Annex Creek penstock photographed in 2017 • The original Salmon Creek penstock photographed in 2016
The Annex Creek penstock was, and the Upper Salmon Creek penstocks is a riveted steel pipeline manufactured by Willamette Iron and Steel Works in Portland Oregon. Willamette manufactured each section of pipe based on the design specifications. There were double and triple riveted pipe sections. The double riveted sections were constructed of thinner metal and used closer to the dams where the head pressure is lower. Tripple riveted sections were constructed of thicker steel and used closer to the power houses where the head pressure is higher. Representative double and triple riveted cross sections are depicted in the figure below.
Riveted pipe detail.
After manufacturing, each section of pipe was shipped from Oregon to the projects where workers moved each section of pipe to its appropriate location and hand riveted the pipeline together. At each project workers constructed a railway and utilized a six-ton locomotive to move material up the mountain sides. The same locomotive was used for both projects.
Railway at Annex Creek • September 1916
Sections of Pipe Staged for Transportation on the Locomotive • Locomotive on Railway• July, 1913
Workers Lowing Pipe into Place • Hand Riveting Sections of Pipe Together • July, 1913
The new Annex Creek penstock is a welded steel pipeline measuring 7,031 feet in length. It has a diameter of 42 inches, with a wall thickness that varies from 0.313” to 0.375”.
The penstock design includes plans and profiles that define the overall layout, configuration, alignment, support methods, pipe sizing, and materials. Detailed fabrication drawings and pipe-laying plans were developed to support construction, along with the manufacturing of the pipe. The project scope also included demolition of the existing penstocks, route clearing and grading, and installation of the new system.
Due to the steep and remote terrain, nearly all heavy lifting was performed using helicopters based in Juneau. Both removal of old pipe sections and installation of new pipe segments were carried out by helicopter.
Penstock Components Staged in Juneau • Summer 2024
Penstock Construction • Helicopter Lifting Section of Pipe • Summer 2024
Finished Penstock • Summer 2025
The original steel girder and cable suspension bridge supported the penstock pipeline as it crosses the gorge of Annex Creek between Upper Annex Lake and Lower Annex Lake. The original bridge was installed in 1915 and remained in service for over 100 years until it was replaced in 2024. The bridge spanned approximately 90 feet above the gorge. The bridge is approximately 90 feet long. It is constructed using 16-inch by 6-inch steel horizontal I-beams with 8 ½-inch by 9 ½-inch vertical steel I-beam risers. Suspended on either side from the vertical risers are 1 ¼-inch steel cables that support the 16-inch horizontal I-beams. On either end of the bridge are concrete buttresses
Bridge Disassembly • Steel Girder and Cable Suspension Bridge • Spring 2024
The new Annex Creek penstock bridge uses two spans that share a precast concrete center pier. The upper span is approximately 52 feet, and the lower span is approximately 33 feet. The spans are supported using steel beams that extend from the center pier to both outer end-piers.
Finished Penstock Bridge • Summer 2025
View the original penstock pipeline bridge plans below.