I thought this was an incredibly cool image, showing the changing path of the Mississippi River (in this case where it meets the Ohio). (via Flowing Data)
When I was a kid, I was fascinated by water flow and erosion. I remember spending a whole day on a woodside hill watching the evolution of an ad hoc stream of water, playing around with damming it in some places, creating new channels, etc. When I went to the beach, I never built castles but attempted to build walls and channels to shape the way the tide flowed. Since I am free associating, I also remember visiting a huge model of the Mississippi, I think near Vicksburg, that I thought at the time was the coolest thing on Earth. Not even sure today if it still exists.