The creation of the lake and its outlet is attributed to a post-glacial isostatic rebound, which resulted in uplift of the land.

It is adjacent to Koli National Park, known for its scenery in summer, and winter skiing; and is near the whitewater rafting centres at Ruunaa Hiking Center, Lieksa and Nurmes.

The catchment of the lake is densely forested, particularly on its rugged western shore, which is capped by Koli hill.

[3][4] The shores have a diverse landscape including barren lands, rock faces, exposed soils and beaches.

The two towns in the basin are Nurmes, at the northern tip of the lake, and Lieksa on the eastern shore, which have a population of 18,700 and 11,500 respectively.

The herbaceous vegetation in marsh areas consists of various grasses (Poaceae), sedge (Carex sp.)

The agricultural crops grown in the area are barley, oat, grass and potato, which are mainly used for feeding domestic animals.

[6] Road 73 passes through the Uimaharju on the southern side of the lake and goes through very scenic spots to the east end.

[7] The climatic data has been recorded at the two major towns in the lake basin, namely the Lieksa and Juuka during the period between 1931 and 1960.

At Lieksa, the average annual rainfall was 583 mm (23.0 in) with mean minimum and maximum temperatures recorded as −7.1 °C (19.2 °F) (December) and 16.4 °C (61.5 °F) (July).

During the same period, the average rainfall at Juuka was 544 mm (21.4 in) and the minimum and maximum temperatures recorded were −7.1 °C (19.2 °F) (December) and 16.1 °C (61.0 °F) (July) respectively.

[3] The phytoplankton species recorded in the lake are Chlamydomonas spp., Monoraphidium confortum, M. dykowskii, Scenedesmus spp., Tabellaria flocculosa, T. fenestrata, Melosira sp., Mallomonas sp., Synura sp., Dinobryon sp., Merismopedia warmingiana, Anabaena flos-aquae and Aphanizomenon gracile.

The benthos recorded in the lake are: Insecta – Dicrotenipeds pulsus, Glyptotendipens spp., Polypedilum pullum, Chaoborus flavicans, Stictochironomus spp., Tanytarsus spp.

[3] Other parameters of chemical and physical properties of the lake waters measured relate to transparency (in meters), pH value, dissolved oxygen, COD, and concentrations of chlorophyll, nitrogen and phosphorus.

It was declared a National Park in 1991, after an intense debate between the environmentalist lobby and the owners of the land property over the location of the Hotel Koli, which stands at the top of its namesake hill.

There are also a marina and swimming beach on the shores of the lake, where a Finnish dancing stage called the huvilava stands.

[9] The Paalasmaa Island is the largest in the lake, with its peak rising to 225 metres (738 ft) above mean sea level.