Curved lines are more dynamic than straight lines. They take you into the image. They create flow. Sometimes they are very obvious (bending river, winding road), other times they are more subtle. From time to time we need to help them just a little to stand out more. Here is another image from Zion National Park to make the point.

There is no huge or prominent S-curve. Yet, due to where more light fell, the depth of the water, and the appearance of rocks there is a subtle and subdued S-Curve (or more accurately, a reverse S-curve). This curve plays an important part in the composition of this image. It leads your eye into the image, curving on the way there.

          When you are out in the field shooting, always look for curves, they make most images more interesting and pleasing to look at. In post processing, do not be afraid to bring out the curve a bit more. In this image I brightened it just a bit more. It was already brighter than the surroundings, but I just helped it ever so slightly.