Don′t forget your tripod! Using a slow shutter speed (in this case, 3.2 seconds) you can allow the water to blur as it moves during the exposure.

When you have a slowly-moving body of water, this gives you an almost glassy reflective surface,
which allows for a wonderful reflection of the background, in this instance the rocky walls of an abandoned quarry.

And, you can sometimes come up with a surprise, as the buildup of exposure time gives a view not seen by the naked eye. In this case, the river seems to be inhabited by a ghost who is watching me work! Can you see him?