Streams are organic machines that use raw energy and turn it into different products — or organisms — as it flows downstream. Streams begin in the hills or mountains as rainfall seeks low areas and groundwater seeps to the surface. Headwater streams are ƒften too small to have fish — the Maryland Biological Stream Survey rarely finds fish in drainage basins of less than 300 acres.

Frequently, these streams are totally shaded by the riparian forest canopy, which limits the growth of algae. Groundwater often keeps temperatures stable. Inhabitants — mainly insects — rely on twigs, leaves and other organic matter for fuel, which they break down into finer particles.

As a stream widens, more sunlight hits the water, increasing the production of algae and stream plants. These, combined with fine particles from upstream, fuel a more diverse community of insects and fish. Warmer temperatures also create thermal niches along the stream that increase species diversity.

As streams widen into rivers, sunlight increases algae production even more, making it the dominant food source. But fine particulates are important, especially in rivers with a lot of sediment, which blocks light and limits algae production.

Under this scenario, known as the River Continuum Concept, the links between the land and the aquatic biological community are strongest in the headwaters and diminish as the stream becomes larger. Land-based activities that affect the headwaters not only alter the stream locally, but can have ripple affects downstream as well