Headwaters are composed of many small, branching streams that form the headwaters, representing most of the total stream length within a watershed. Leaves, fruit, branches and whole trees of stream side forests fall into these headwater streams. Fungi and bacteria grow on the surface of this detritus and begin to break down the complex cellulose molecules of dead plant material. As shredders gnaw through wood and leaf, they create fine particulate organic matter that is filtered from the flowing water by collectors. This group of organisms, invertebrates that spin fine capture nets or are equipped with long bristles, can collect these small suspended particles coated with biofilm communities.
Collectors build up nutrients in their own biomass or fecal pellets.
These nutrients thus become available as food for predators. Energy laden nutrients, originally derived from riparian forests, are eaten and released and eaten again as they spiral downstream. These nutrients flow from headwaters to mid-reach streams where other aquatic communities await. Nutrients flowing from mid-reach to lower river reaches are harvested by a different community of collectors. Excess nutrients finally flow out of river mouths to enrich estuaries.