Waterfalls, whether big or small are not only beautiful, but they greatly improve the health of a river or stream by aerating the water with oxygen. Without turbulent mixing at the air/water interface, the dissolved oxygen (DO) content in the water could drop below a critical threshold needed by many aquatic critters like fish to survive. Since rivers are always flowing in one direction, interruptions in aeration upstream can result in reduced DO levels downstream. This is one of many reasons why the introduction of dams or other man-made structures that alter the natural flow of a river must be considered with great scrutiny before being built.