any spring, stream, or river viewed as an ecosystem. The waters are flowing (lotic) and exhibit a longitudinal gradation in temperatures, concentration of dissolved material, turbidity, and atmospheric gases, from the source to the mouth. There are two major zones: rapids, shallow water where currents are strong enough to keep the bottom clear and firm; and pools, deeper waters where currents are reduced and silt and other debris collect on the bottom. Each zone has its specially adapted life forms.