The dam on the river Mahanadi is the longest dam in the world.Hirakud Damis built across theMahanadi River, about 15 km fromSambalpurin the state ofOrissainIndia. Built in 1957, the dam is one of the world's longestearthen dams.Behind the dam extends a lake, Hirakud Reservoir, 55 km long. Hirakud Dam is the longest man-made dam in the world, about 26 km in length.