You have three ways you can produce a sense of motion in your picture of any moving subject:
1) Pan with the moving subject;
2) Use a high shutter-speed to freeze the action; and
3) Use a slow shutter-speed to blur the action.
Which technique is best depends upon the particular subject. Your subject is running water. Which technique is best for it?
Generally, panning isn’t the best technique to use for water because a stream of water doesn’t give you a single subject on which to focus and pan. So your choice of technique is reduced to either a high shutter-speed or a low shutter-speed. Which is best depends upon the effect you want to create.
For example, if you want to capture the action of crashing waves in your photograph, a high shutter-speed can be effective because every droplet is suspended in mid-air, producing a sense of the power of the waves. On the other hand, if you want to capture the motion of a babbling brook in your picture, our choice would be a slow shutter-speed. How slow? This depends upon a number of factors, one of which is the speed of the water. Since you can’t measure the water-speed, you have no choice but to decide this by trial and error. Bracket a series of low-speed shots, starting with 1-second, and progressing through 1/2-second, 1/4, 1/8, 1/15, and 1/30 – and decide which is best. The slow shutter-speed will transform the water into slivers of “angel hair.” The effect can be wondrous. Of course, be sure to use a tripod when you use a slow shutter speed.
What about photographing a waterfall? Either technique works effectively, depending on the effect you want. The high shutter-speed captures the droplets in mid-air, and bespeaks the power of the water. The slow shutter-speed transforms the water into a shimmering cascade of angel hair. Our suggestion: Try both. See which one you like best!