Langinkoski is a rapid located near the mouth of the Lankila branch of the Kymi River. It is a shallow area with many rocks below and above the surface. The calm current arrives from the north-east and turns south towards the Gulf of Finland. The calm river is transformed into a violent rapid as the water hits the rocks and is squeezed trough the narrower part of the river. The river once again becomes calm after passing through the bridge over the rapids, after which the river wides and flows to the Baltic Sea.

Before the bridge, the rapid already calms as the area gets wider. However, towards the bridge the river once again narrows and causes a more violent current for one last time before the river ends.