Taken from the top of the Croton Dam, looking down onto the spillway. You can clearly see its dual nature: part natural, part man made. On the left the water rushes through the natural part, and on the right it flows over the man made part.
As an article on the Amusing Planet website states:
The dam features an unusual stepped spillway, located to the side of the retaining wall and is partly natural and partly man-made. Water flows down the natural portion in wild waterfall-like rapids, while at the man-made portion water spills down huge steps.