Rainbow Falls, on the Wailuku River just outside of Hilo. As Rainbow Falls was on our way to work, I stopped in there fairly frequently. Because of this I have four different pictures of them, all at different flow levels. One, just after a huge rainstorm, has water spanning the entire rock face. Another, after a prolonged dry spell, has a single trickle of water coming out.
If you look really closely you can see a rainbow in the mist. Morning is the best time to catch rainbows, as the sun is directly on the falls.