What we can do is use a fast shutter speed to freeze the movement of the water when, for example, it breaks over rocks, or when the crest of a wave is blown away by a strong wind, or as it slips like a glossy sheet over the lip of a waterfall. We can also select a slow shutter speed and record the scene with motion blur. Some photographers use a neutral density filter to get a speed sufficiently slow to make the water look like ice or fog. But a dark day, a small aperture and a low ISO will also do the trick, though with less blur. When we do this we allow the viewer to see that which the eye normally cannot, and in this respect, we produce images akin to those we make with a macro lens, which also reveals things we normally don't see.
My photograph shows the River Ribble in spate at Langcliffe, North Yorkshire, though it could have been taken on any fast moving, shallow, rocky river. Looking down from a bridge I selected a diagonal composition with undulating water and waves produced by barely covered stones, and used the "dark day" technique noted above. The highlights on the surface of the river have produced trails that give a sense of turbulent flow that a higher shutter speed wouldn't have done. I quite like the effect! You might like to compare it with a faster shutter speed used on a much slower moving river here.
photograph & text (c) T. Boughen
Camera: Olympus E510
Mode: Aperture Priority
Focal Length: 83mm (166mm/35mm equiv.)
F No: f10
Shutter Speed: 1/8
ISO: 100
Exposure Compensation: 0 EV
Image Stabilisation: On