Novosibirsk Reservoir or Novosibirskoye Reservoir, informally called the Ob Sea, is the largest artificial lake in Novosibirsk Oblast and Altai Krai created by a dam on the Ob River near Novosibirsk. The dam, built in 1956, is for generating hydroelectric power. The reservoir is 160 km long and up to 20 km wide. The power of hydroelectric power is 400 MW. During the summer, it is one of the most popular destinations for Novosibirsk residents with many yachts and boats dotting the surface of the lake, and the beaches teeming with people.

http://en.wikipedia.org/wiki/Ob_Sea