Hot spring fed. So only lake at that elevation you can swim in.
Never been there but Dave Pallides spoke about it on his CanAm missing 411 project. Always carry a personal locator beacon folks. People mysteriously go missing all the time in parts like this especially in places with the word Devil in them.
Great trail, pack light, and lots of water. The extra distance from Jasper Lake to Devils thumb lake is so worth it.
Another beautiful lake along indian peaks southern loop. There is camping here, however, to camp you are supposed to get a permit. If hiking the loop, i recommend staying at Jasper or here to get to the CDT as early as possible. The top of the pass gets pretty narrow. Can be sketchy for those afraid of heights. There are bushes to hold onto if you need to help you along. No bathrooms, no campfires. Fantastic views!