They are often amazing repositories of a wide range of life. Fish and aquatic plants such as algae live in streams. All sorts of creatures we don't see live there too. Microscopic bacteria and other tiny organisms live naturally in streams along with the macroinvertebrates . These creatures all play important roles in the food chain of a stream community.
Healthy streams support a wide variety of organisms.
In order to support them, a stream needs a variety of habitat areas. Some aquatic organisms need the high oxygen levels which can be found in riffles . Some depend on pools, snags, and bends for the way such areas accumulate food. In faster moving parts of a stream, the surface area of the cobbles are needed for shelter as well. Fine sediment should not cover the substrate of a healthy stream.