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A river or stream changes greatly from its source to the mouth, this also affects the types of organisms that live in it. Near the source the river is usually clear, cold and has a swift current which doesn't allow much to accumulate on the bottom an it also does not allow phytoplankton to grow. The main producers are algae which are attached to rocks or organic material that is carried into the river or stream. Insects are common and the trout are the most commonly found fish. Most organisms at this part of a river locate their food by sight (Campbell 2001).

Downstream, the current of the river slows and the base widens. Marshes and other wetlands are common here. Sediments and phytoplankton suspended in the water cause it to be warmer than upstream. Worms and insects that burrow into the sediment on the river bed are common. Waterfowl, frogs and catfish are also common in the downstream part of a river. Most of the organisms find food by senses rather than sight because the water is usually to murky because of suspended sediments (Campbell 2001).

In a river or stream phytoplankton and seaweed are usually the main producers, while small organisms like minnows, snails or crayfish feed on them and larger fish like carp or salmon feed on them, which are the predators of the ecosytem (The Watershed Ecosystem).

An example of specific streams is the Ohio primary headwater habitat streams. In these streams there are a wide variety of organisms that make up the ecosystem in the rivers. There are many different kinds of fish such as the Creek Chub, Rainbow Trout, and Largemouth Bass. There are also many macroinvertabrates like Snails, Worms, and Water Mites.

A specific example of a habitat that an organism living in a watershed needs is the Rainbow Trout. Trout prefer cold water streams which causes them to usually live in the head water of the streams. They also live in the fast moving currents like the current of the head of a river or stream.A Rainbow Trout feeds on many things, but mostly insects but it will eat anything that swims by it or floats by it that is small enough for it to eat. Rainbow Trout also feed on fish eggs and flies (Rainbow Trout Facts).