The length of the river is 74 km (46 mi). The area of its basin is 393 km² (152 mi²). The Sestra River flows over the Karelian Isthmus. It used to fall into the Gulf of Finland until the early 18th century. After the construction of a dam for the needs of the munitions factory in Sestroretsk, a part of the river was turned into a reservoir called Sestroretsky Razliv (Sestroretsk Overflow) two meters (six feet) in depth with an area of 10.6 km² (4 mi²). Since then, the Sestra River has been flowing into this reservoir. The Sestroretsk Overflow is separated from the Gulf of Finland with a ridge of artificial sand dunes. Excess water is dumped into the Gulf of Finland through a canal, which is 4.8 km (3 mi) long. The Sestra River served as a natural border between Russia and Sweden 1323–1617 and between Russia and Finland in 1812–1940.