Shooting a flowing stream or a waterfall with an exposure time of a second or more will give you white water from the blurred movement. This gives a smoothed appearance to the water, showing the movement in an unusual way.
But to create this effect successfully you will need to prepare. Firstly you should use a tripod, or at the very least somewhere to rest your camera as you are aiming for a slow shutter speed.
The Canon PowerShot G, S, SX ranges along with the IXUS 310 HS and IXUS 300 HS feature Shutter speed-priority AE (Tv) mode. If you own one of these, then switch to Tv mode and select a speed of one second or more to achieve the desired level of the effect.
Alternatively, try slow shutter speeds when you are shooting in a rain shower. Each individual raindrop will travel a short distance during the exposure, giving elongated streaks of water. The best effects are obtained by shooting into the light, so that the raindrops are lit from behind. Again, do remember to use a tripod or stable location to rest your camera.