Best Shutter Speed for Waterfalls
It's easy to get addicted to the silky-smooth water that comes from leaving your shutter open for several seconds. However, this effect is overused almost to the degree of garish HDR, to the point where it is nothing more than a gimmick.
On the other hand, it can be distracting and unnatural to have water in your image frozen like ice. So, where is the happy medium?
Short answer: It depends.
Long answer: You need to go by the effect you want. For waterfalls, leaving the shutter open for more than a second will mean that the entire waterfall is silky smooth, without any texture. For an ocean scene, leaving the shutter open for a while (usually ten seconds or longer) can turn the waves to mist. Below are examples of both (the top at six seconds, the bottom at thirty seconds):
This effect works well for some photos, but it is also overused. Instead, for a lot of photos, it can be best to leave some texture in the water. Doing so all depends upon how fast the water is moving. In a slow-moving ocean, you can leave your shutter open for a few seconds. The example below was shot with a six-second shutter:
On the other hand, with a fast-moving stream or waterfall, something more in the 1/4 to 1/3 range could be best. For example, the next shot was taken at 1/3 second, and it shows a good amount of texture:
I think that 1/2 second to 1/4 second would be a good go-to shutter speed range for rivers and waterfalls, assuming that those settings work in your lighting. I don't like to kill detail with a slow speed, and I don't like to freeze the water unnaturally with a fast one.
However, there is a time and a place for everything, which I hope becomes clear with these final two photos: