Lunzer Untersee (Lower Lunz Lake) is of glacial origin, and located approximately 120 km southwest of Vienna. There are two small neighboring lakes, Mittelsee (Middle Lake) and Obersee (Upper Lake) in the upstream of the Seebach, the main inflowing river to Untersee. The lake is only 0.7 km2 wide, has a maximum depth of 33.7 m, and a fairly short residence time (0.7 year). The nutritional level of the lake water is still oligotrophic, but the littoral zone has recently been eutrophicated considerably, with remarkable changes in the aquatic flora.

Ninety percent or more of the total drainage basin area consist of karstic plateau and the forests of Norway spruce and European beech. The density of year-round population in the drainage basin is low (11 km-2), but summer visitors may amount to 8,000 day-1 at the maximum. Domestic waste water of this district has been diverted to preserve the quality of the lake water.