The lake normally stays on the Cerknica plain for about nine months a year. Water usually spreads over 20 km2, but at its fullest, the lake covers a surface of nearly 30 km2, making it the largest lake in Slovenia. During the summer drought, however the water covers only 0,1 km2.

While draining is a long  process, the lake can fill up in just a couple of days in autumn when there is heavy rain. It also fills up in the spring, when the snow melts.

276 bird species have been observed in the area, and the place is the home to 45 mammal species, 125 butterfly species and 15 amphibian species.

The native flora has  adapted to the complicated dynamics of the changing water levels. Three species of carnivorous plants can be found in the marshes, and amphibian plants thrive in water as well as on land. The wet meadows offer almost unique  growing conditions to many endangered species.