Patrick, Great picture of a calm sea, but not to worry, the drain plug is still in place. I see curved patterns, but I do not see a complete swirl as you imply. My guess it is flotsam common on the lake surface blown by very mild surface breezes into the pattern you see. Sometimes on calm days you can see a near-circular pattern of clear fresh water floating (yes, floating) on the surface of the lake delineated by the material floating on the salt brine next to the fresh water. That fresh, and cold, water comes from a spring near the eastern shore under the lake. A strong breeze near the surface destroys the visible boundary and soon mixes the two densities of water. Ed