There's been some very nice shots of waterfalls posted over the last year in LWITG. [http://www.renderosity.com/viewed.ez?galleryid=1185086]. I can't remember all the photographers' names, so hopefully they will jump in here with comments. Working on an overcast day seems to be the real key trick to prevent the water from washing out. Then, as indicated, use a low ISO value, and maybe a filter, to allow increasing the exposure time. Many of the digital cameras do not allow long exposures. Can always put the camera on a tripod and take several frames, then combine them with software.