The stream biocies consist most typically of inhabitants of the riffles and sand bottom pools found throughout the course of a river. The riffle and pool organisms therefore make up the two different portions of the stream community.
Living things in a river can be thought of as belonging to three main categories: the fixed or sedentary organisms, the drifting life, and the active swimmers. Plants are not abundant in the stream biocies although the upper surfaces of rocks and riffles may be completely covered with branched filamentous algae (particularly cladophora) and a few species of water mosses. Diatoms, mostly cessel forms, may be numerous in early spring and again in the autumn. Some of the minute diatoms attach themselves by gellatinous housings and some desmids live in attached tubes as do some rotofers and midge larvae.
Dominance in the true sense, such as occurs in terrestrial communities, does not exist in streams, although the algae and mosses passively provide food and shelter for the active forms of various forms of organisms. Plankton is mostly absent in swift running water, but may be abundant in sluggish pond-like stretches of large rivers.
Animals of a stream must be able to maintain positions in spite of the current. If not strong swimmers, they may have flattened or streamlined bodies with broad surfaces for adhesion or strong claws for clinging to the rocks.
An example of these types are the nymphs of may flies and stone flies, snails and flatworms. Other animals may have spinning glands to make threads for attachment, for example black flies and buffalo gnats. Or they may have strongly adhesive brushes of hair while others may have suckers or sucker-like bodies, like some may fly nymphs or beetle larvae. Finally, others may have bodies that are adapted for burrowing into the bottom such as some may fly nymphs and dragon fly nymphs. Whatever their adaptation for withstanding currents, they all have to subsist upon whatever the river affords.
So far as they do not feed on each other they must be able to catch what flows by. They may have as parts of their bodies filter fans or they may spin nets to spread against the current, such as done by the caddis larvae. Others may have scraping devices such as snails or may fly nymphs, but in general they are usually predators and detritus feeders.
Among the fish that are normally encountered in swift streams are trout darters, black nose dace, and some suckers. Some of the warmer water fish, minnows, sun fish, bass, carp and catfish, are also inhabitants of rivers or streams.
Streams that empty into the ocean may have a special fauna of anadermous (upstream) fish such as salmon, chad, stripped bass and some trout. These fish spend much of their life in the sea, but migrate into fresh water streams to spawn.