Kárahnjúkar Hydropower Plant, officially called Fljótsdalur Power Station is a hydroelectric power plant in Fljótsdalshérað municipality in eastern Iceland, designed to produce 4,600 gigawatt-hours (17,000 TJ) annually for Alcoa's Fjarðaál aluminum smelter 75 kilometres (47 mi) to the east in Reyðarfjörður. With the installed capacity of 690 megawatts (930,000 hp), the plant is the largest power plant in Iceland. The project, named after the nearby Kárahnjúkar mountains, involves damming the rivers Jökulsá á Dal and Jökulsá í Fljótsdal with five dams, creating three reservoirs. Water from the reservoirs is diverted through 73 kilometres (45 mi) of underground water tunnels and down a 420-metre (1,380 ft) vertical penstock towards a single underground power station. The smelter became fully operational in 2008 and the hydropower project was completed in 2009.

Alcoa Corporation is a Pittsburgh-based industrial corporation. It is the world's eighth-largest producer of aluminum. Alcoa conducts operations in 10 countries. Alcoa is a major producer of primary aluminum, fabricated aluminum, and alumina combined, through its active and growing participation in all major aspects of the industry: technology, mining, refining, smelting, fabricating, and recycling.