Annex Creek drains a watershed on the south flank of Annex Ridge and flows generally southeast for 5 miles (8 km) through the Annex Lakes to the western shore of Taku Inlet, about 0.35 miles (0.6 km) southwest of the Annex Creek hydroelectric power facility and 12 miles (19 km) east of Juneau, Alaska. Upper Annex Lake is about 2 miles (3.2 km) long at an elevation of 844 feet (257 m), and Lower Annex Lake is about 0.5 miles (0.8 km) long at an elevation of 620 feet (189 m). The local name was first published by the U.S. Geological Survey in 1916 and adopted by the U.S. Forest Service in 1929. The bedrock underlying the Annex Creek watershed is granodiorite and tonalite which are intrusive rocks of the Coast Plutonic Complex, the largest granitic outcropping in North America. The intrusive rocks were formed during volcanic activity along the Coast Range Arc that resulted from the subduction of the Kula Plate along the western margin of the North American Plate. Volcanism in the arc began during the Late Cretaceous period, or about 100 million years ago, and the granodiorite of Annex Creek was intruded in the Paleocene, or 66 million years ago. Volcanism began to decline along the length of the arc about 60 million years ago when movement of the Kula Plate shifted from a subduction plate boundary to a transform plate boundary with motion parallel to the Pacific Northwest coast. The magma intrusions were mostly in marine sediments that were intensely heated resulting in metamorphism and localized mineralization. In 1880, only 13 years after the Alaska Purchase, gold was discovered in Juneau at what is now known as Gold Creek. The early mining efforts were placer mines that used large water jets to wash gravel through sluices to separate the gold. Most of the placer mining along Gold Creek in the Silver Bow Basin was finished by the 1890s, and the miners’ attention was drawn to the lode, or hard-rock gold deposits.
Miners soon discovered that the gold ore around Juneau was relatively low grade and to be profitable mining had to be done on a very large scale. The smaller mining operations in the Juneau area began consolidating and by 1911 there were only three large mines operating including the Treadwell Mine on Douglas Island, the Alaska Juneau or AJ Mine, with a mill operating near downtown Juneau, and the Alaska Gastineau Mine operating in the community of Thane near Sheep Creek. Thane is named after Bartlett L. Thane who was an American mining engineer who pioneered hydroelectric power in Juneau. He first came to Alaska in 1897 and was hired by Herman T. Tripp to work the summer at the Sumdum Chief Mine in Endicott Arm. He then returned to California to complete his studies in mining engineering at the University of California at Berkeley. He returned to Alaska and by 1903 he was the first superintendent of the Eagle River Mining Company before becoming Managing Director of the Alaska-Gastineau Mining Company. The gold ore around Juneau was relatively low grade and mines yielded an ounce of gold from 10-20 tons of ore. To run a profitable operation, mining had to be done very efficiently and on a very large scale. By 1911, Bart Thane had acquired control of the Perseverance Mine and developed a plan to operate it with very large volumes of ore. His expansion plans required a significant amount of power and Harry L. Wollenberg was hired to prepare a report on the feasibility of constructing a hydroelectric project on Salmon Creek to provide power for a new mine and mill. By 1915, the Alaska Gastineau Mill was built on Gastineau Channel south of Juneau and a hydroelectric power project was built at Salmon Creek to power the mill. The rock crushers, conveyors, and other plant equipment were all powered with electric motors. Salmon Creek was generating 6,000 horsepower and the mill was so efficient that with additional equipment it could process 12,000 tons of ore per day. To do this they also would need another 6,000 horsepower of electric power. Wollenberg searched for another hydroelectric project and soon began investigating Upper Annex Lake.
Tapping Upper Annex Lake and transmitting the power to Juneau were challenging projects since the lake was located in a steep and rugged valley, and powerlines would have to cross Sheep Mountain and survive powerful winter storms. Two small rail lines were built to support these projects, one running along Annex Creek and one ascending Carlson Creek. The railway at Annex Creek had a floating dock at the power plant, a cabled-powered incline running up the steep mountain, and a level section of rail reaching from the head of the incline to the lake with a total length of 1.5 miles (2.4 km). A wooden dam 15 feet (4.6 m) high was built to raise the lake level and water storage capacity and a tunnel 1481 feet (451 m) long was blasted 150 feet (46 m) below the surface and connected by a pipe to the powerplant at sea level. Crews began constructing steel towers and stringing power lines up Carlson Creek towards Juneau at the same time the power plant was being built. A narrow-gauge tramway about 5 miles (8 km) long was constructed alongside the creek to haul materials to the various construction camps. The power from Annex Creek allowed Thane’s Gastineau operation to briefly become the highest capacity mill in the world. Mining engineers originally expected the value of the gold in the mine to be $1.50 per ton of ore. Their goal was to be able to mine and mill the ore for 75 cents per ton and have a very profitable operation. They met their goal for production costs, which varied from 68 to 77 cents per ton. But the assay value of the ore actually mined was far lower than expected, averaging around $1 per ton with a recovery of 84 cents per ton. The Gastineau Mine could not be made profitable and was shut down just six years later in 1921. During its operating life, from 1916 through 1920, the Gastineau mine produced 471,000 ounces of gold valued at $9.7 million at the time. The wooden dam at Upper Annex Lake failed in 1935 and was rebuilt, then replaced in 1967 with a cement dam. In 1944, the property was leased to Alaska Electric Light and Power then sold outright in 1973. Today, the Annex Creek Hydroelectric Project still provides about 5 percent of Juneau’s electric energy demand. Read more here and here. Explore more of Annex Creek and Taku Inlet here:
