Neutral Density Filters.

I hope you get some interesting results, Rod.

As you say, the need to use a tripod is off-putting, which is why I prefer to hand-hold the camera, often with the help of a stabilised lens to minimise the shake. Hand-held exposure times cannot be longer than about 1/8 second with a wide-angle lens and image stabiliser, but this is still long enough to get a pleasantly smoothed effect with running water. I think the noise issue I mentioned previously would only show up with exposure times of many seconds.