Water in lakes is constantly being replaced.
To replace all the water in a lake takes a long time.
It takes 70 years to replace all the water in Lake Vättern.
For Lake Mälaren it only takes three years.
The difference in time depends on the size of the lake, the depth
and the amount of what that runs in.
Just like running water,
a lake reflects its surroundings.
Streams and rivers are influenced by the landscape
which they travel through,
which also has significance for the water quality of the lake.