The Leonharder See is situated in the basin of Villach, embedded in a gravel terrace that was stored up over east- alpine old crystallines during the retreat of the glacier. Its basin is a former dead ice hole. The shores are flat, there is a little reed and macrophyte population.
On the northern banks there is a complex of hotels and a shore wall prevents the coming up of natural vegetation. In the south agriculturally used areas reach down as far as the lake. In the south- east there is a public bath with a large rest- cure meadow. All the other shore zones are used as private bathing beaches that mostly have been silted up.
The runoff of the Vassacher See passes a flat moor region and flows into the Leonharder See after a flowing distance of about 300 m. The runoff on the east shore zone has only little water volume.
Stocking with no permission. In 2000 grass carp were stocked by the regional fishing association without any allowance. This caused an obvious worsening of the water quality together with an increase of the algae bio-mass and a decrease of the visibility depth. Grass carp eat water plants when temperatures reach more than 18 °C. By secretion the nutrients that were bound to the water plants before, get available for different algae species. So the wanted reduction of plants cannot be reached, the situation even got worse. According to the Carinthian Fishing Law stocking of non-native grass carp in public waters is forbidden or needs allowance.