First of all, sorry that I've been away from the blog for a week. I have a book deadline at the end of this month and just haven't had any free time. The book is on the subject of how to win photo contests though and I'll write more about it later this week. But right now, onward to one of my favorite tricks: turning waterfalls and streams into a flowing ribbon of water.
I'm sure you've seen a lot of waterfall photos done this way and creating the effect is very simple: To turn flowing water into those pretty white ribbons of motion, just set your camera to the shutter-priority exposure mode and select a longish exposure. The actual length of the exposure depends on how fast the water (or any other subject, for that matter) is moving and how much you want it to blur. I find that most moderately-fast moving streams and waterfalls blur nicely between 1/2 and one full second. Any longer than that and you run into a situation of diminishing returns: a blur is a blur is a blur once it's maxed out. In other words, the blur doesn't get any better or more dramatic and, in fact, can get too soft looking.
I think it's better if you actually cut back on the length of the exposure a bit once you reach that point because then you also get some detail in slower moving areas (like after the water hits the pool at the bottom in this case--you can actually see some detail in the foam, etc.). I exposed this shot for 1/2 second at f/20, for example, but also shot some at 2 seconds and 4 seconds and I thought the water was too soft and blurry, so I sped up the shutter speed to 1/2 second. Again, it really depends on the speed of the motion and just how much of a blur you want.
By the way, you may get frustrated when working in bright light that you simply can't select a shutter speed that slow without the photo overexposing. That's because your camera doesn't have an aperture small enough to balance out the long exposure. One solution for that is to use a neutral-density filter over the lens. These filters reduce the light hitting the sensor without changing its color. Cokin sells some that I think are reasonably priced. I'll write more about them in a future posting. For now, just do a Google search on "neutral density filters" and you'll find plent of info.