Rivers, lakes and waterfalls make interesting photographic subjects but they are not without challenges. When photographing these subjects, watch for bright sunlight reflecting off the water. While it can often add to the image, it can also cause exposure difficulties and you may have to choose between losing shadow detail or having the reflections wash out.
Fortunately, on a digital camera, you can try both and see how they look. A polarizing filter can often be used to control reflection. In optimal situations, you can often choose between the water surface appearing mirror-like or clear simply by rotating the polarizing filter. When photographing moving water, your choice of shutter speed can dramatically alter the image. A fast shutter speed ‘freezes’ the water, while a slow shutter speed makes it appear smooth. Landscape photographers often use a tripod, ND filter, polarizing filter, small apertures (f22), a slow shutter speed and a slow film (or low ISO setting) to produce stunning photos of waterfalls with smooth, even surreal looking water.