Gröntjärn Nature Reserve is visited foremost for its geological value and its unusual hydrological conditions. The reserve lies in a narrow valley surrounded by mountains, about 5 km west of the village Västerstråsjö, 25 km northwest of Delsbo.
Formed during the ice melting
The ground in the area is part of a peculiar ice river sediment, which was formed when the ice sheet melted about 8500 years ago. There are many hollows in the area, so called kettle holes, which are traces of ice blocks buried below sand and gravel. When the ice blocks melted the ground subsided and formed these steep sided hollows. Many of the hollows are now filled with water and the most notable of these are lake Gröntjärn.
Spectacular water level variations
There are no streams that provide the lake with water. This is because it depends mostly on the rainwater finding its way to the groundwater through permeable ground. It is the groundwater that fills several of the lakes in the area. Gröntjärn’s beautiful turquoise green colour is an effect of the groundwater, which is free from particles unlike the water in streams.
Gröntjärn is interesting because of its large water level variations. The difference between the highest and lowest water level is more than 13 metres. This is due to a complicated interaction between suface water, groundwater and the groundwater flow.
Spring pasque flower and kidney vetch
The flora is adapted to the dry conditions in the area. Pine trees, berry bushes and lichens cover large parts of the reserve. Spring pasque flower is unusually common by the kettle holes in the area. On Gröntjärn’s shores you will find a poor but interesting flora with among others the glacial relic, an alpine catchfly as well as kidney vetch.