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 annually for Alcoa’s Fjarðaál aluminum smelter 75 kilometers to the east in Reyðarfjörður. With the installed capacity of 690 megawatts, 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 kilometers of underground water tunnels and down a 420-meter vertical pen-stock towards a single underground power station. The smelter became fully operational in 2008 and the hydro-power project was completed in 2009.
The Kárahnjúkar Dam is the centerpiece of the five dams and the largest of its type in Europe, standing 193 meters tall with a length of 730 meters and comprising 8.5 million cubic meters of material.
The project has been heavily criticized for its environmental impact and its use of foreign workers.