Sixteen lakes held up by something that is still growing
The barriers between the lakes are not rock that was already there. They are being laid down now, by moss and algae, at roughly thirteen and a half millimetres a year.
Everyone photographs the water. The structure holding it is easier to miss: a staircase of sixteen lakes, each one dammed by a wall the water itself is building.
Plitvice was proclaimed a national park on 8 April 1949, the first in Croatia, and inscribed by UNESCO on 26 October 1979. Neither date is when the lakes were made. That work has not stopped.
The wall is a deposit
Water leaving the plateau carries dissolved calcium carbonate. Where it runs over moss and algae the carbonate comes out of solution and sets on them; the organism dies inside the deposit and the deposit keeps its shape. The moss that does most of this work has a name — Palustriella commutata — and it is doing it right now.
The barriers gain about 13.5 millimetres a year. The one closing Kozjak manages 5.6. The barrier at Prošćansko runs three times faster than that.
How old the lakes actually are
The active barriers are between 6,000 and 7,000 years old. The lakes are a Holocene structure — younger than farming, younger than writing. The valley holds older ones too, from two earlier warm periods: 90,000 to 130,000 years ago, and 250,000 to 300,000.
The number that decides everything
Three conditions have to hold at once for tufa to form: a saturation index above 3, a pH above 8.0, and dissolved organic matter below 10 milligrams of carbon per litre.
The first two are geology and they take care of themselves. The third is not geology. Organic load is what wastewater and crowds add, and above that line the deposition simply stops.
It has come off the map once already
Plitvice went onto the UNESCO list on 26 October 1979 and onto the List of World Heritage in Danger in 1992. It came off again in 1997. The park itself is older than both — 8 April 1949, the first in Croatia.
The cap, and what it is for
Since 2019 entry has been capped at 1,200 visitors an hour. The park first counted a millionth visitor in a single year in 2011. The cap is not about queues at the boardwalk. It is about condition three.
Two claims rest on the park’s own publications alone: the 1,200 visitors-per-hour cap and the millionth visitor in 2011.
What is not settled
Whether the water is currently above or below the 10 mg/L line is not established here. The park runs its own monitoring; those figures were not obtained for this piece. Until they are, the honest statement is that the threshold exists and that visitor numbers feed into it — not that the barriers have stopped growing.