Puujärvi is a six-kilometre-long “esker lake”. It is assumed to have been created when meltwater from the Ice Age moved large land masses. The lake was created behind the esker 15 metres higher than Lohjanjärvi lake, into which Puujärvi’s waters drained. The esker is one of the continuations of the Salpausselkä ridges that runs towards Hanko.
The average depth of the lake is 8.5 metres and it is 23 metres at its deepest. The lake is not part of any body of water and it takes over 8 years for the lake to turn over fully.
Some of the water in the lake is groundwater and protected by the NATURA 2000 conservation programme. Water quality is monitored annually by authorities and the results are published in the water conservation association’s Puujärvi bulletin.
There is a 10km/h speed limit on the lake.