When I went to one of my favorite places recently, my aim was to use some long exposures photographing a stream running through a gorge. The place that I went is Padley gorge in the Peak district. You can get magical lighting there when the sun shines. As there a lot of trees down in the gorge you get some wonderful dappled light effects.
Because the day I chose to go was heavily overcast and the trees were blocking much of the light, it allowed longer exposures, the exposures were about 5 seconds. The camera was set on 100 ISO at f22. If the light is too bright for long exposures use neutral density filter to cut out some of the light. On exposures of this length you will need to use a tripod, here a light travel tripod was used with a rock in bag hanging from it for added stability.
The effect that you get on the water is where the water is running fast it goes almost milky. On the slower running water the effect is that it looks like a sheet of glass. This type of photography is well worth giving a try and experimenting with.