I have a Canon Digital EOS Rebel XT camera, and I wonder how you could take photos of moving water like rivers, waterfalls and fountains...
If you want the flowy dreamy look, try to use a longer shutter speed and a tripod It depends on the water flow rate. In the sun, you may need to use a ND filter as stopping down will get you into diffraction-limited range.
For some waterfalls (particularly violent ones), you may want the opposite: a very fast shutter speed.
You will need to experiment and instantly review with each waterfall as the flow rates are different. If you are getting distracting reflections, a polarizer will also help (and act as a bit of a ND filter by eating light)
The most common approach to taking great flowing water pictures is to use a long exposure. This allows the "soft, dreamy flow" of water to be captured as you have probably seen in many photos. Achieving a long exposure may require extra equipment, depending on how the scene is lit.
To achieve a long exposure, you will need to reduce the amount of light reaching your sensor, and expose the sensor for a long duration of time. This usually requires a very tight aperture, such as f/11 or higher, and a long exposure, anywhere from 1/6th of a second to several seconds. You should always use the lowest ISO setting your camera is capable of, as this helps in extending your exposure times. Normally this is ISO100, however if your camera goes to ISO50 or lower, use the lowest setting you can achieve.
In addition to your camera exposure settings, you will need to use a tripod to stabilize the shot. Lengthy exposures in the large fractional seconds to full seconds can not be stabilized by hand, even with the most powerful IS/VR. A tripod will not only allow you to stabilize your shot, but they often have levels that help you keep your shot horizontally level.
To further minimize camera shake, a couple other tools can help. If your camera supports a Mirror Lockup feature, use it. This allows you to flip the mirror with the first press of the shutter, and take the shot with the second. Sadly, pressing the shutter often causes greater camera shake than the mirror flip itself, so an important tool is a remote or cable shutter release. These provide a remote controlled or cable-connected shutter button that allow you to flip the mirror and release the shutter without touching the camera, eliminating camera shake.
Tuning for the Scene
Which settings you use will be dependent upon the scene. Brightly lit scenes may require a very tight aperture. I would avoid using an aperture tighter than f/22, as around there your image will start showing noticeable distortion due to diffraction. I would recommend keeping your aperture between f/11 and f/16, and if you need a longer exposure, add an ND (neutral density) filter to the front of your camera. An ND filter will reduce the amount of light that enters your camera, allowing you to expose for a longer duration.
The longer the shutter is open, the more it will capture that soft sense of motion in water. The amount of motion you wish to capture can greatly affect your scene, so the longest exposure time is not always the most appropriate. A "shorter" long exposure, anywhere from 1/6th of a second up to perhaps 1 second, will show the motion and shape of water better. A "longer" long exposure, up to several seconds, will smooth out the shape of water and enhance that "soft, dreamy glow" effect. The longer you expose, the glassier flat, clear parts of the waters surface will get.
Dimming and Deepening
I highly recommend investing in some ND filters if you wish to do long-exposure shots of water. Even if a scene does not explicitly need an ND filter, they help to mitigate the volume of light and give you more control. ND filters can help deepen the contrast and vibrance of a scene, and can add to the overall effect.
Beyond just photographing waterfalls, longer exposures can create a nicer effect for a wide variety of water scenes. From simple streams or rivers, to lakes lakes, to expansive ocean coastline shots, or tighter shots of reefs and tidal pool areas around coast lines. Longer exposures of water can smooth out the noisy detail of ripples and waves on the surface, reducing the complexity of a scene, helping users zero in on the true subject you are trying to portray.
There are different ways that you can shoot moving water:
The best time for each effect of course varies a bit depending on the scene, and the focal length.
A polarising filter has great effect on water, as light that bounces off water gets polarised. You can use it either to remove reflections, or enhance them.
A neutral (ND) filter is useful if you want to get a longer exposure time. A tripod (or any other means of keeping the camera still) is needed for the longer times.
Camera : Nikon D5000. Lens : 18-55 VR (kit)
In order to get a "dreamy" flow look I chose shutter-priority mode, set the time to 1/50 (I didn't have the tripod with me so I couldn't hold the camera stable for more time) and shot this first picture.
In order to capture the bubbles of water I set the shutter speed to the fastest value allowed by my camera. However I had to increase the exposure compensation in order to get the image a bit lighter.