Zelenci is a nature reserve which is known by a resurgence springs of Sava Dolinka River. (Sava River has two sources branches: Sava Bohinjka and Sava Dolinka). The majority of water flows underground from Tamar valley where is the first spring - Nadiza - white waters of Nadiza. Lake of Zelenci has intensive green color (that is how it got its name - 'zelen = green'). Present lake is situated on the edge of a swamp which is a remainder of the greater lake which was created after the last glacial period. In that time torrents from the Karavanke Alps heaped up rubble which made a barrage for the waters flowing from the West - so the lake (Podkorelško jezero) was accumulated.
Temperature of the lake is constant (5~8 °C) in all seasons.
Area of the nature reserve is 14 hectares; the deepest part of the lake is 3 m.