|Karst lake, Ponor.
|As far as we know this information was accurate when it was published (see years in brackets), but may have changed since then.
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Lake Ioannina seems to be a karst lake. Unfortunatley we had no scientific literature about this, but a view on the location, and a second one on the map tells pretty much.
The lake has numerous tributaries, but no visible drain. Drainage is through karst caves under the lake. We could read this in several guide books, but it is pretty obvious. If the lake had no drain at all, it would grow until it over flowed its banks. Then there were a drain. The only alternative is less rain than evaporation, but this would make it a salt lake pretty soon.
This lake is no seasonal lake, it has more or less the same level all the year. The reason is not known to us, but a guess is, that the caves provide a sort of overflow channel, and drain the lake in winter, when it gets lots of water from the surrounding mountains. In summer most of the tributaries fall dry and the subterranean drainage probably too.
The lake looks pretty nice from the mountains around. It has a clean blue colour and a bath in the lake seems to be a good idea. Unfortunately the lake is rather dirty, the shores are full of reed and water snakes. In some books the missing drain is blamed for this situation, but that is obviously a mistake. The lake has a drain, which is subterranean, and it is definitely not responsible for the poor water quality! But the city of Ioannina, with nearly 100,000 inhabitants, uses the lake for its sewage for centuries now.