I think you're probably right about the dark area being a little too big. This kind of shot is an awkward one and one that i always have a whole lot of trouble with.
The area is beautiful and it is crying out to be photographed, but once the camera is out of the bag, how the bl@@dy hell do you frame it. Nine times out of ten you choose an image that you are not happy with just because you feel that it should be good because the area was so nice and you can't believe that you didn't do it justice in the picture.....or atleast i do!
As it is i'm not the biggest fan, mostly because of the composition. The (my) eye wanders around although it finds the waterfall, it gets drawn to the water below the dark area in the bottom left corner, which it shouldn't. There is too much nice subtle lines of light to be wasted in that dark area, so i have cropped it.....
I now feel slightly more comfortable looking around the image, (i would prefer not to have so much at th top as the shape of the image is slightly awkward, but left the clouds in and the point is the same) the subtle yellow light guides me in from the bottom right towards the waterfall, which pulls me to the tree and with it's shape leads me back round into the image, going up to the cloud and then back down the tree on the right to start the journey again. Now my eye feels contained, and with it's containment a certain contentment, especially with the extra details of the image such as the snowcapped 'hills'.