Clear Lake is the headwaters of the McKenzie River. The lake is primarily fed by snow runoff from nearby Mount Washington and the surrounding areas. The runoff filters through underground caverns for more than 20 years before emptying into Clear Lake. The lake is also fed by two small creeks, which may dry up seasonally, along with Great Spring, the source of the McKenzie River. Clear Lake is reported to be one of the most exceptional freshwater dive spots in Oregon. Submerged 100 feet (30 m) deep in the lake is a stand of upright trees that were killed approximately 3,000 years ago when volcanic activity created the lake. The trees are remarkably preserved due to the cold year-round water temperatures of between 35–43 °F (1.6–6 °C).

It is a shame that this image can't show you just how clear the water is, you can see the bottom clearly.