The Salmon Creek Dam is a concrete arch dam on the Salmon Creek, northwest of Juneau, Alaska. Built in 1914, it is the world's first constant-angle arch variable radius dam. Since it was built, over 100 such dams have been constructed all over the world. The dam was designated as a National Historic Civil Engineering Landmark by the American Society of Civil Engineers in 2022.
The dam was built by the Alaska-Gastineau Mining Company to meet the electrical energy needs for mining operations. The dam continues to be fully functional for hydroelectric generation, as one of the drinking water sources to Juneau city and for aquaculture and fishing. When built, adoption of the constant arch design for the dam reduced costs by 20% because less concrete was needed to construct the dam. Of the two hydroelectric power stations built at the initial stage (one at the upper level and the other at the lower level) – the latter one is still in use after a new powerhouse was built adjoining the old one – it produces 10% of the energy needs of Juneau city. When built, the dam and its two power plants were considered engineering wonders. Both are operated and maintained by the Alaska Electric Light & Power (AEL&P).
Topography
The dam was built in a forested, scenic and narrow valley of the Salmon creek, which runs from Salmon Creek Reservoir and flows southwest for to the Gastineau Channel. The dam (marked as Juneau B-2 in USGS maps) is located at the terminus of the tram-way that was built specifically by the Alaska Gastineau Mining Co. The tram-way was built for hauling material for constructing the dam and its associated power stations. The power supply was for the mines located at a site to the south at Sheep Creek. However, a long road had been built by AEL&P to the upper powerhouse at the base of the Salmon Creek Dam, which has since been de-commissioned.
The creek runs for a length of within the watershed, which has a creek divide of and a ridge line of . The stream bed has large gravel and bedrock substrata and its gradient decreases downstream of the dam. The average width of the stream is and depth of water is about . The basin is surrounded by hills with steep slopes and with elevation above above sea level. The distance from the dam to Juneau city is , to the Juneau harbor is , and to the Juneau Harbor Seaplane Base is .
Evolution of the arch dam design
During the Roman period, the theory of building a curved dam (arch dam) was known as a means to withstand the water pressure and hold the masonry joints. However, in the background of masonry arch dams which dominated dam building scenario in the 19th century and with introduction of concrete technology for building dams, the structural design of arch dams underwent a dramatic change in its economic evolution to minimize use of construction material and inter alia, the cost of construction. Basically, an arch dam is a structure that curves upstream and the water pressure is transferred either directly to the valley sides or indirectly through concrete abutments. Theoretically, the ideal constant angle arch in a "V"-shaped valley for such an arch dam has a central angle of 133° of curvature. This theory led to the development of the "constant-angle" (or variable radius) arch dam, which was also thinner in design. The theory was first initiated in North America for several dams, and in Alaska in particular, in 1913 with the building of the Salmon Creek Dam, which was completed in 1914. In this regard, Bartlett Lee Thane, the mining engineer, who made a lasting impact in the mining industry – in the Alaska-Gastineau Mining Company – was instrumental in introducing this design of thin arch dam with help from his former football team mates. Lars Jorgensen evolved the specific first design of the constant arch dam. However, AEL&P gives design credit to their then Chief Engineer Harry L. Wallenberg for the Salmon Creek Dam. The idea for building such a dam had been thought about 30 years earlier to economize on the use of construction material for building the dam. However, it was only in 1913 that the concept was transformed on ground by the pioneer mining engineer Thane.