When you first see it, it looks like a perfect rainbow conforming to the contours of the river, trapped inside a waterfall. But then your brain realises that that's not possible. How are there colours in the water? How is there a rainbow at night? It's the trick of long exposure photography. And I can't stop looking at it.
Called 'Neon Luminance', Sean Lenz and Kristoffer Abildgaard of From the Lenz teamed up to create rainbows in the water by dropping Cyalume glow sticks into waterfalls in Northern California. They set their cameras for a longer exposure (30 seconds to seven minutes in some cases) and were able to capture the path of the glowstick through the river. It's a beautiful effect.
You can see more of their pictures here.