We said that the last post was the final in the series about Planina polje.
This is Unica river, the sixth apparition of the “River of the Seven Names” flowing serenely across the plain. But can you tell in which direction is it flowing?
From the hills onto the plain, from the forest to the meadow – right? Yeah, no.
Unica is a Karstic river, and Karst is strange. Planinsko polje was born in the middle Miocene when the Idrija fault cracked open and the rocks were mixed up. The surrounding hills and plateaus are made of Cretaceous and Jurassic limestone, while the bottom of the polje (and Jakovica hill) has some Triassic dolomite. The limestone is permeable, all the water there can only flow underground, while the dolomite is not – so when the subterranean river hits the polje, it emerges vehemently from under a sheer cliff, fully formed and magnificent.
Planina polje on which Unica flows is a flat-bottomed closed depression, so it meanders as hell. And when it gets closer to the northern end of the polje and the limestone hills above it, things begin to happen rather fast.
Unica forks – some say humans have helped it do that – and approaches the limestone plateau from two directions. When it reaches the cliffs of Škofji lom and Pod stenami, it disappears underground in a series of sinkholes (ponors, to be exact).
But those ponors can gulp only so much water, so with the spring thaw and the autumn rains Unica comes with a vengeance, overwhelmes the sinks and floods the whole polje.
Now people living at the edge of Planina polje did not appreciate their fields being under water for months. They were looking at the ponors into which Unica was disappearing, and had an idea. What if we make the sinks bigger, so they could take more water, and reduce the floods – maybe even get completely rid of them?
The idea became flesh during the Austro-Hungarian Empire, and they did it meticulously. They employed the father of modern speleology Wilhelm Putick, the first to seriously explore, research, and describe peculiar Karst phenomena of Ljubljanica basin.
He came up with this:
The ponors of Unica were cleaned, enlarged, encased in concrete and protected by a metal mesh. Those retouched sinks are called katavotrons, and are still pretty much the same as they were when Putick designed them in 1889.
And it worked – kinda. Karst is hard to tame, and most human endeavours to change and “better” it ended mostly in failure. The floods did subside (at least the locals say so), but not much. Planina polje did not become an agricultural paradise; the meadows are still wet, the soil poor, and regular floods still cover the plain once or twice a year.
But people don’t really like to concede defeat when fighting Nature – so they went straight to the other extreme: if we can’t make the floods go away forever, maybe we can make them permanent? Let’s turn Planina polje into Planina lake!
This plan had nothing to do with tourism, fishing, swimming, or leisure boating – it was about generating electricity. A hydro plant, you say?
No – a nuclear one.
In the 1960s Slovenia and Croatia (while still republics in socialist Yugoslavia) started to flirt with nuclear energy. and in 1970 they entered a contract to build a nuclear power plant. Two, in fact, one in each republic. But where?
A nuclear power plant needs a river nearby to cool it. And the only river in both Slovenia and Croatia that has a flow large enough and stable enough for a nuclear power plant is Sava (and maybe Drava). Experts identified a couple of suitable places; one in Croatia near Prevlaka, and two in Slovenia: Krško, and Dolsko near Ljubljana.
But Sava at Dolsko is still a young river, moody and unpredictable. To guarantee a suitable and stable flow for cooling a nuclear power plant, another source of water would be needed. And the idea was to dam Unica, turn Planina polje into a resevoir, drill a 10 km long tunnel straight to Ljubljana Marshes, and enlarge the riverbed of Ljubljanica – so the flow of Sava at Dolsko would be within desired parameters.
Yes, you are right – it is a rather crazy idea. You can’t make an impermeable dam on limestone; the water will always find a way around or under it, just like it happened when people tried to dam Stržen on Cerknica polje.
In the end the nuclear plant was built near Krško, the idea of flooding Planina polje was abandoned, and this textbook example of a Karst polje was saved. Today Planina polje is a Natura 2000 site, a beautiful place with rare flora and fauna, a veritable menagerie of Karst phenomena, one of the most picturesque places in Slovenia – and a prime kite flying spot!
Kite aerial photos shot with Nikon P330 on Cindy delta, except the meandering Unica (Insta 360 on The Rokker), and the flooded polje (Nikon P330 on The Great White delta)