Firstly a nicely composed image and with a rainbow - a bonus.
It's a bit over exposed and a pity about the burnt out area at the end of the rainbow. Not an easy subject with bright water and dark areas. I would guess it was a sunny day, it's better to photograph waterfalls on a cloudy day. If you expose for the main part of the waterfall you could then bring up the darker areas up in Photoshop. Do you have a highlight screen on your camera? If you have, you can see where areas are burnt out and then make adjustments. Take this kind of shot in manual
As I said a nice composition but try again when the sun is not shining on the water.
Hope this helps a little!