Clear Lake is fed by snow run-off that passes through miles of volcanic rock, coming into the lake virtually pure. It is estimated that it takes about ten years for the snow and glacier water from the Sisters to percolate through the volcanic rock to perk up in Clear Lake as springs. And being a few degrees above freezing, it doesn't harbor much in the way of algae. A hike around the north end of the lake brings you to this treasure: Great Springs, where the clear water just flows out of the rock, to the delight of the butterflies that land there to drink.