"You might not realize this, but the visible surface waters we see in a river are moving quickly and that's a big problem for rivers. Riffles, rapids, and turbulence stir oxygen into the water so it's oxygen-rich, but at the same time the water is a relative dead zone because it's either nutrient-poor or else the nutrients are flowing downstream and out of the system. Then, if you dig down, the bed of rocks, mud, and debris that a river sits on are also largely devoid of life." To read on: Lukas Guides: The River's Liver, 2/29/24