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At Zelenci, water from the underground stream Nadiža (originating in the Planica valley) re-emerges through the porous bottom of a two-m deep lake, whose waters are noted for their deep, brilliant green. The spring and its surrounding area are named after this colour, zelen meaning "green" in Slovene and "Zelenci" being its possessive plural.)
The Upper Sava Valley is the result of action by the Planica glacier, creeping from beneath Jalovec, Ponce and Mojstrovka. The area contains many lake sediments, suggesting that Zelenci is a remnant of a once much larger Lake Koren, created by the retreat of the glacier, which carried along much debris. In retreat, the bulk of it was deposited at what is now Podkoren, damming the Sava with the Koren Pass, which hemmed in glacial melt and formed an extensive lake. The Sava then tunneled through this natural dam, lowering the water level of lake Koren until only Zelenci and its surrounding wetlands were left.
Zelenci is considered the beginning of the longer of the two sources of the Sava, the longest Slovenian river at 221 km. The spring is actually the re-emergence of an underground creek called Nadiža, whose first source is near the mountain hut at Tamar, but which spends most of its course underground after disappearing at the Ledine gravel basin near Rateče.
The porous chalk of the Zelenci lakebed permits a constant upwelling of groundwater in the form of tiny jets, a phenomenon unique in Slovenia. The lake formed by the springs has a constant year-round temperature of 5–6 °C. From the lake, the water flows into a stream, which empties eastward into the "Blata" ("Muds"), a 200 m wide and one km long shallow marshland. The actual riverbed of the Sava Dolinka does not begin until after this, at Podkoren beside the slopes of Mount Vitranc.
Zelenci was declared a nature reserve in 1992, both for its geological interest and as home to numerous endangered animal and plant species. The area of the nature reserve is 47 ha. The maintained paths feature informational signage, as well as viewing bridges and an observation tower.
The unspoiled natural beauty of the multiple springs and emerald-green lake had also drawn the attention of the painter Ladislau Benesch.
- Marsh Lousewort (Pedicularis palustris) (Slovene: močvirski ušivec)
- Buckbean (Menyanthes trifoliata) (Slovene: navadni mrzličnik)
- Common cottongrass (Eriophorum angustifolium) (Slovene: ozkolistni munec)
- Marsh arrowgrass (Triglochin palustris) (Slovene: močvirska triroglja)
- Water crowfoot (Batrachium sp) (Slovene: vodna zlatica)
Besides trout, which inhabit the lakes and feed on mayfly (Ephemeroptera) and stonefly (Plecoptera) larvae, the area boasts several endangered species on the Red List of Endangered Animal Species of Slovenia:
- Common rosefinch (Carpodacus erythrinus) (Slovene: škrlatec)
- Whiskered bat (Myotis mystacinus) (Slovene: brkati netopir)
- Viviparous lizard (Lacerta vivipara) (Slovene: živorodna kuščarica)
- European adder (Vipera berus) (Slovene: navadni gad)
- Sand lizard (Lacerta agilis) (Slovene: martinček)
- Decree on Declaration of Zelenci as Nature Reserve, Official List Nos. 53/92, 17/94, Official List of Upper Carniola, No. 32/96 (Odlok o razglasitvi Zelencev za naravni rezervat, Uradni list št. 53/92, 17/94, Uradni vestnik Gorenjske, št. 32/96)
- Sinfo (Jan 2006) ISSN 1854 0805 http://www.ukom.gov.si/fileadmin/ukom.gov.si/pageuploads/Sinfo/2006-januar.pdf
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- "Reke, dolge nad 25 km, in njihova padavinska območja" [Rivers, longer than 25 km, and their catchment areas] (in Slovene, English). Statistical Office of the Republic of Slovenia. 2002.
- "Sinfo" (pdf). Jan 2006. Retrieved 2008-10-25.