I took this shot on our hike back from Mirror Lake in Yosemite. The time of day was after 4:00 PM. The sun had moved in the Valley and the river was in the shadow of Half Dome. This created a low setting allowing me to set my shutter to a slower speed. It was also dark enough I could leave the aperture open to get a better depth of field adjustment.
The key to making the water look silky is to work with a slow shutter speed. In some of these shots I had the shutter open for as long as 8 seconds. Two crucial pieces of equipment will be needed for these long exposure shots. One is a tripod so the camera will remain stable and stable. Two, you will need a cable release or remote release. The will eliminate and unwanted movement on the camera during exposure.
The best time is to shoot these types of images either early in the morning or later at night. Another good way is to take the image during overcast or cloudy days. Remember the key is to slow the shutter speed. If it is too bright or sunny sometimes you can trick the camera by closing the aperture. Keep in mind that the smaller the aperture opening the greater the depth of field (focal length).
This is the same shot. I cropped the image tighter. I straightened the image so the sight line would be level. In Photoshop CS3 I made the image warmer (not as blue or cold). I also did a little post sharpening.