This flickr pool is highly recommended. Found it while searching pictures from Prince William Forest Park, which led me to this nice picture:
- Of steps behind a dam built by the CCC during the construction of Prince William Forest Park. If the image is from the spot I think it is, then the pool is filled by diversion of water from a stream and returns to the stream a bit further downstream (that is, it takes what was a C-shaped bend and makes it a D-shaped pair of flows). Originally intended for recreation, the pool now exists only as a piece of the 'natural' hydrology.
Image by flickr user kewlwhlz.