Lost Lake is a lake in the Willamette National Forest, in Oregon, in the United States. It is known for the way that the water level changes during the year. There are lava tubes in the bottom of the lake. During the summer, the water drains out of the lake through these tubes. In some years, this leaves the lake with very little water, turning it mostly into a meadow. Starting in the autumn the lake begins to fill again. This happens as the amount of water coming in to the lake from streams and melting snow is higher than that flowing out. By spring, it has its highest water level. 