Lost Lake, near Bend, Oregon, is really only a part-time lake. In the summer, it’s a meadow! Lost Lake is connected to a lava tube that drains the lake annually, but not the same time every year, and not at the same rate. This year, the drainage happened in April and went pretty fast, as you can see in this video from The Bulletin.
The drainage stops when the lake is iced up, and the rest of the year, the state of the lake depends on the rate of inflow vs. the rate of drainage. You can read a more comprehensive explanation at mental_floss.