Annex Creek flows southeast for 5 miles (8 km) through the Annex Lakes to 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. It starts at elevations of approximately 2500 feet (762 m) and drains a watershed on the southern flank of Annex Ridge. Upper Annex Lake is approximately 2 miles (3.2 km) long with an elevation of 844 feet (257 m), while Lower Annex Lake is 0.5 miles (0.8 km) long with 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 consists of granodiorite and tonalite, which are intrusive rocks of the Coast Plutonic Complex—the largest granitic outcropping in North America. These rocks were formed during volcanic activity along the Coast Range Arc, resulting 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, approximately 100 million years ago. The granodiorite of Annex Creek was intruded in the Paleocene, around 66 million years ago. Volcanic activity started to decline about 60 million years ago when the Kula Plate’s movement changed from a subduction to a transform boundary, shifting parallel to the Pacific Northwest coast. Magma intrusions primarily occurred in marine sediments, causing intense heating, metamorphism, and localized mineralization. In 1880, just 13 years after the Alaska Purchase, gold was discovered in Juneau at what is now known as Gold Creek. Early mining efforts involved placer mining, which used large water jets to wash gravel through sluices to separate the gold. By the 1890s, most placer mining along Gold Creek in the Silver Bow Basin had concluded, shifting miners’ focus to lode, or hard-rock, gold deposits.

By 1911, miners in the Juneau area had discovered that the gold ore was relatively low grade, requiring large-scale operations to be profitable. Smaller mining operations began consolidating, leaving only three large mines: the Treadwell Mine on Douglas Island, the Alaska Juneau (AJ) Mine with a mill near downtown, and the Alaska Gastineau Mine in Thane near Sheep Creek, named after Bartlett L. Thane. An American mining engineer, Thane pioneered hydroelectric power in Juneau. He first arrived in Alaska in 1897, hired by Herman T. Tripp to work at the Sumdum Chief Mine in Endicott Arm. Thane returned to California to complete his mining engineering studies at the University of California, Berkeley. By 1903, he was the first superintendent of the Eagle River Mining Company and later became Managing Director of the Alaska-Gastineau Mining Company. The gold ore around Juneau yielded approximately one ounce of gold per 10-20 tons (9-18 tonnes) of ore. To run a profitable operation, mining had to be done efficiently and on a large scale. By 1911, Bart Thane had acquired control of the Perseverance Mine and developed plans to operate it with high volumes of ore. His expansion required significant power, so Harry L. Wollenberg was hired to assess the feasibility of constructing a hydroelectric project on Salmon Creek to power a new mine and mill. By 1915, the Alaska Gastineau Mill was built on Gastineau Channel, south of Juneau, and a hydroelectric project was constructed at Salmon Creek to power the mill. The rock crushers, conveyors, and other equipment were powered by electric motors. Salmon Creek generated 6,000 horsepower (4,474 kW), and the mill was so efficient that, with additional equipment, it could process 12,000 tons (10,886 metric tons) of ore per day. This required another 6,000 horsepower (4,474 kW). Wollenberg began investigating Upper Annex Lake for an additional hydroelectric project.

Tapping Upper Annex Lake and transmitting power to Juneau were challenging projects due to the lake’s steep, rugged valley location. Power lines needed to cross Sheep Mountain and withstand powerful winter storms. Two small rail lines supported these projects: one along Annex Creek and another ascending Carlson Creek. The Annex Creek railway featured a floating dock at the power plant, a cable-powered incline up the steep mountain, and a level rail section extending from the incline’s head to the lake, totaling 1.5 miles (2.4 km). A wooden dam, 15 feet (4.6 m) high, increased the lake’s level and water storage capacity. A tunnel, 1,481 feet (451 m) long, was blasted 150 feet (46 m) below the surface and connected by a pipe to the power plant at sea level. Simultaneously, crews erected steel towers and strung power lines up Carlson Creek towards Juneau. A narrow-gauge tramway, about 5 miles (8 km) long, was built alongside the creek to transport materials to 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 initially estimated the gold value in the mine at $1.50 per ton of ore. Their goal was to mine and mill the ore for 75 cents per ton to ensure profitability. They achieved production costs ranging from 68 to 77 cents per ton, but the assay value of the mined ore was significantly lower than expected, averaging around $1 per ton with a recovery of 84 cents per ton. Consequently, the Gastineau Mine could not be made profitable and shut down in 1921, only six years after opening in 1916. During its operational period, the mine produced 471,000 ounces (13,354 kg) of gold, valued at $9.7 million at the time. The wooden dam at Upper Annex Lake failed in 1935, was rebuilt, and replaced with a cement dam in 1967. In 1944, the property was leased to Alaska Electric Light and Power, then sold outright in 1973. Today, the Annex Creek Hydroelectric Project supplies about 5% of Juneau’s electricity demand. Read more here and here. Explore more of Annex Creek and Taku Inlet here: