Slåensø

Slåensø is a lake located immediately south of Borre Lake in Silkeborg South Forest and is usually counted among the Silkeborg lakes, although it is only connected to Borre Lake by a drain at Millingbæk. The lake has been dammed up by about 2 metres in order to harness the water power that once powered a bone-mill, a sawmill and a wood-pulp factory at the outlet to Millingbæk.

Slåensø is about 1 km in a west-easterly direction and about 275 m at its widest point in a north-south direction. It is fed only by a series of ochre springs on the south side, and marshy red deposits can be seen at their outlets in several places. With its limited inflow, the lake has very clean water. The springs have also given the lake its name. It was originally called Slaugen Lake, where "Slaugen" comes from the Old Norse "Slagna", meaning "to spring".

The lake is located in a very hilly terrain and the lake reaches a depth of up to 12 metres. It is a popular excursion destination with the Kongestolen immediately south of Slåensø, from where there is a fine view to the north over the lake as well as Borresø. There is also a good walking path around the whole lake and a parking area by the road a few hundred metres from the eastern end of the lake.

Together with Grane Langsø, Slåensø is Denmark's cleanest lake according to the programme Detektor, judged on water clarity and phosphorus and phytoplankton content.