Eklutna River
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The Eklutna River is a 22 mile (35 km) long river located in the Southcentral region of the U.S. state of Alaska. A degraded anadromous stream of glacial origin, it originates at Eklutna Glacier and flows through Eklutna Lake and a canyon up to 350 feet (107 m) deep, emptying into the Knik Arm of Cook Inlet approximately 17 miles (27 km) northeast of Anchorage. It has one significant tributary, Thunderbird Creek, which enters the south bank about 1 km (0.6 mi) upstream from where the river exits the canyon and forms a sediment fan. Due to water impoundments on the Eklutna River for power generation, Thunderbird Creek is currently the main source of water in the river.
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[edit] The lower dam
Anchorage was largely electrified by the late 1920s, but as demand increased, Eklutna was selected as a power source because of the hydroelectric potential of the river, then known as Eklutna Creek, and Eklutna Lake. In 1927, the City of Anchorage contracted with the Anchorage Light and Power Company to construct what is now called the Old Eklutna Hydroplant. Construction included a low-head storage dam at the outlet of Eklutna Lake and a 68-foot (21 m) high concrete arch diversion dam (known as the Lower Dam) in the river canyon 8 miles (13 km) downstream of the lake. The diversion dam diverted water through a 1/4 mile-long tunnel to a turbine house near the village of Eklutna. Since its construction, the Lower Eklutna Dam has been a barrier to fish movement upstream. When the Upper Dam was brought online, the Lower Dam was shut down as a result and the Lower Eklutna Dam was allowed to fill with gravel. This dam is no longer operational and is currently completely backfilled with sediment to a depth of approximately 68 feet (21 m) at the upstream face of the dam.
[edit] The upper dam
Military expansion in Anchorage during the 1940s stressed the capacity of the Eklutna power generation system and it was upgraded several times. In 1948 the Bureau of Reclamation recommended the construction of a new dam to raise the level of Eklutna Lake to an elevation of 875 feet (267 m) above sea level with a tunnel intake at 830 feet (250 m). Construction was completed in 1955. The new system replaced the aging storage dam at the lake outlet with a new dam that diverted water through a 4.5-mile (7.2 km) long, 9-foot (2.7 m) diameter concrete lined tunnel with a capacity of 640 cubic feet (18 m³) per second (18 m³/s) to a turbine house on the south bank of the Knik River. The dam, as modified, is an earth- and rock-filled structure, 555 feet (169 m) long and contains approximately 5,000 cubic yards (3,800 m³) of material. This new plant used essentially the entire storage capacity of Eklutna Lake and no water was made available to operate the existing plant at Eklutna.
The 1964 Good Friday Earthquake severely damaged the dam. Because of this, a new storage dam was built downstream from the existing storage dam at the lake outlet. The new Eklutna Dam (referred to as the Upper Dam) is an earth and rockfill structure 815 feet (248 m) long and 51 feet (16 m) high containing 85,000 yd³ (65,000 m³) of material. The spillway is a rectangular concrete conduit through the dam with an uncontrolled overflow crest. The maximum capacity of the spillway is 3,315 ft³/s (94 m³/s). There are no outlet works through the dam, as the power tunnel serves in that capacity. As the Upper Dam impounds 100% of the flow from Eklutna Lake, the river's volume immediately downstream is zero.
