Pictures do not handle contrast as well as the human eye, and you will notice parts of your photo are much brighter than others. See the photo of Beamers Falls. This isn't always possible to control, but you may greatly reduce the amount of obstructions by simply waiting for a few minutes. Break the above rule - when you feel that a hiker or building will vastly improve the viewer's idea of scale. Photos from the bottom of the falls usually impart a more impressive quality.
Photos taken from the top of the falls can make the falls look less spectacular than it really is.
Pictures from the side of a falls can be confusing. Standing in, or very close to the water can help. If you can't get too close to a falls, use the rule of thirds. Break the photo scene into a grid that is 3 by 3. Place the waterfall in one of the corner grids.