How does the lake in my back yard stay so clean?

I live in Kansas and in my back yard there is a small lake about the size of a swimming pool. If I swirl it up with my hand it will be cloudy but it settles within 24 hours to clear and stays clear. It does not smell bad and I don't see birds or animals around it. Additionally it is not near any drains and I see very little water bugs. Why does it just stay clear?