Leading lines
The Leading Lines rule states that the human eye is drawn into a photo along lines. The lines can be curves, straight, diagonal, or otherwise. The lines bring the viewer in and take them wherever you want the attention to be. If your picture doesn't have lines, you need to have something else to let the viewer know where to look. In this photo, you have the water falling leading you down. Then at the bottom of the water, you have the line leading across the photo.