A clean river nurtures plankton and waterweeds, which are the food and nourishment of small fish. This natural cycle of life becomes a feature of any stretch of water; even an artificially created piece of water like a pond or a canal will, if left alone, start its own “life cycle”, beginning with things that wriggle in the mud. In a short time, small predators will move in, and a complex little food chain will have started. All water, whether still or flowing, is richly productive, for where there is water there is life. You often see people fishing in a narrow water channel; they fish in ponds the size of puddles, in streams that look like gutters. And they are nearly always rewarded with a catch. The combination of water and organic matter can always be counted upon to produce some kind of living matter. The one condition is that the water must have a certain proportion of oxygen, for all life needs some oxygen. In other words, it must not be polluted with chemicals.