In the middle of the town, the Logaščica stream disappears into the Jačka sinkhole. At first it runs in a shallow bed on the bottom of Logaško Polje and approximately 300 metres before reaching the sinkholes, it descends into the 20-metre deep and narrow Jačka valley. This ends with a vertical wall where there are several netted sinkholes. The water then flows out to the western springs of the Ljubljanica and Vrhnika.
Sinkholes absorb water during light rain, but when the karst underground is filled up after a heavy rainfall, water starts stagnating and the level quickly rises. The last great flood of which traces are still visible on near-by houses was in 1979. After that year, a barrier was built at the water source to hold back a flood wave during heavy rain and to prevent flooding.