This pattern is fairly common when there is karstic geology, but also happens in other cases: https://en.wikipedia.org/wiki/Subterranean_river https://en.wikipe

This pattern is fairly common when there is karstic geology, but also happens in other cases: https://en.wikipedia.org/wiki/Subterranean_river https://en.wikipedia.org/wiki/Subterranean_river I don't know how common seasonal variation is (per the other comment about the Danube doing a similar thing to the French river).