All creatures need oxygen to survive. Waterways gain oxygen from the atmosphere when water flows over riffles (rocks and stones in the water). Dissolved oxygen enters water through the air, from here it slowly diffuses across the water’s surface from the surrounding atmosphere, or be mixed in quickly through aeration, whether natural or man-made. The aeration of water can be caused by wind (creating waves), rapids, waterfalls, ground water discharge or other forms of running water. A man-made cause of aeration is a waterwheel. Dissolved oxygen is also produced as a waste product of photosynthesis from plants, algae and seaweed.