The Manicouagan impact is the fifth largest confirmed impact crater on Earth, and one of the oldest at about 215 million years. Its a complex crater with outer ring about 100 km in diameter and an inner about 70 km in diameter formed by the impact of a 5 km diameter asteroid. The inner ring is now an annular lake, the fifth largest reservoir in the world, formed by the Daniel-Johnson dam. The Daniel-Johnson dam is the largest multiple arch dam in the world. Multiple arch dams were first designed by John S. Eastwood around the beginning of the 20th century. They substitute form for mass and are able to hold back large amounts of water with less mass and reinforced than other designs because of the inherent strength of the arches they utilize.