The formations at Slunj, Plitvice Lakes National Park, Pamukkale, and (we guess) Kuang Si Waterfall and the Caves in Laos are made of Travertine. Travertine is formed through a strange sort of "reverse erosion." The water becomes super-saturated with calcium as it dissolves the limestone it flows over. When the water is relatively still, it continues to hold onto it's calcium, but as it flows over the edge of a waterfall or other disturbance, it is unable to hold as much calcium and the calcium is deposited on the lip of the waterfall. Thus the lip of the waterfall is built up over time with the flow of water, rather than being eroded away. Incredible!
This seems to explain the beautiful pools in and around Kuang Si Waterfall in Laos, the white pools at Pamukkale, the lakes in Plitvice Lakes National Park, and the way a town could be perched atop a waterfall as Slunj is.