Kárahnjúkar Hydropower Plant (), officially called Fljótsdalur Power Station () is a hydroelectric power plant in Fljótsdalshérað municipality in eastern Iceland, designed to produce annually for Alcoa's Fjarðaál aluminum smelter to the east in Reyðarfjörður. With the installed capacity of , 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 of underground water tunnels and down a vertical penstock towards a single underground power station. The smelter became fully operational in 2008 and the hydropower project was completed in 2009.