Fast or swift flowing streams are roughly all those whose velocity of flow is 50 cm/sec or higher. At this velocity, the current will remove all particles less than 5 mm in diameter and will leave behind the stony bottom. The fast stream is often a series of two essentially different, but interrelated habitats; the turbulent riffle and the quiet pool.
The waters of the pool are influenced by processes occurring in the rapids above and likewise, the water in the rapids are of course influenced by the conditions in the pool. Although pools and riffles are more or less distinct, there are minimal differences between the animals that inhabit them. Some animals that live in the riffles are carried by the current to the pools. Others move back and forth between the two at will as do trout.
The riffles are the sites of primary production in the stream. Many small algal species are epiphytes and grow on the tops of or in among other algae. In the riffles, the aufwichs (attached plants) assume dominance and occupy a position of the same importance as phytoplankton of lakes and ponds. The aufwichs consist chiefly of diatoms, blue-green and green algae and water moss.
Extensive stands of algae grow over rocks and rubble on the streambed and form a slippery covering familiar and often dangerous to fishermen and others who have to wade the stream. Growth during favorable periods may be so rapid that the stream bottom is covered within ten days or less.
The outstanding feature of much of this algal growth is its ephimeral nature. The scouring action of water and the debris carried by the water tear away larger algal growth, epiphytes and sends it downstream. As a result, there is a constant contribution from the upstream to downstream sequence.
Above and below the riffles are the pools. In the pools, the environment differs in chemistry, intensity of current and depth. Just as the riffles are the sites of organic production, so the pools are sites of organic decomposition. Since the velocity of the current is reduced enough to allow a part of the suspended load to settle out, the pools are the catch basins of organic materials.
Overall production in a stream is influenced in part by the nature of the bottom. Pools with sandy bottoms are the least productive because they offer very little substrate for either aufwichs or animals.
Bedrock, although a solid substrate, is so exposed to torrential currents, that only the most tenacious organisms can maintain themselves. Food production decreases as the particles become larger or smaller than rubble.
Gravel and rubble bottoms support the most abundant life for several reasons. They have the greatest surface area for the aufwichs. They provide many protected areas for insect larvae and are the most stable.
Insect larvae differ in abundance on the several substrates. Mayfly nymphs are most abundant on the rubble, Caddis fly larvae on bedrock and diptera larvae on bedrock and gravel.
The width of a stream can also influence overall production. For instance, bottom production in streams 20 feet wide decreases by around one-half from the sides to the center and in streams 100 feet wide it decreases by about one-third. Streams 6 feet or less in width are roughly 4 times as rich in bottom fauna as those 19 to 24 feet wide. This is one reason why headwater streams make such excellent trout nurseries.
For these, the riffles furnish food and the pools shelter. A good trout stream should be about 50% pools and 50% riffles, but the majority of the stream inhabitants live in the riffles on the underside of rubble where they are sheltered from the flowing currents.
Characteristic of the riffle life are the nymphs of may flies, caddis flies, true flies, stone flies, and alder flies or dobsons. In the pools, the dominant insects are the burrowing mayfly nymphs, dragon fly and damsel flies and the water striders.
A major reason for the rich aquatic life in the riffles is the current. Stream animals depend upon flowing current to aid their respiration and to bring them food. Many riffle fauna may suffocate in a few hours when transplanted to still water. This happens even though still water may contain a sufficient amount of oxygen to meet their needs.
Apparently in still water, these organisms becomes surrounded by a closely adhering film of liquid that forms a cloak or blanket of impoverished nutritive substances including oxygen. In fast water, such a cloak does not form, so the absorbing and respiratory surfaces are in continuous contact with the oxygenated streamwater.
A streamline body form offers lessened resistance to water currents. This shape is typical of many animals of fast streams (such as the black nose dace and the brook trout). This shape is also exhibited in some insects. May flies of the genus Baetis which are very fish-like in form are able to move from stone to stone. Other insect larvae possess extremely flattened bodies and broad, flat limbs. This helps the larvae cling to the undersurface of stones. Typical among these are the may flies of the genera Stenomema. Similarly flattened body shapes are the stone fly nymphs and an even more extreme example is the larvae of the water penny beetle.
Other organisms attach themselves temporarily in one way or another to the substrate. The black fly larvae Simulium occur in such numbers on the down current sides of stones that they have gained the name “black moss”.
They attach themselves to rocks by means of a circlet of outwardly directed hooks. Some species have portable houses, the weight of which increases with the velocity of the current. The larvae of certain species of caddis flies construct cases of sand or small pebbles which protect them from the bedload of the current.
Other species have cases firmly cemented to the sides and the bottoms of stones. The thickened walls act as ballast to hold the case on the bottom. The net spinning caddis fly firmly cement to stones funnel shaped nets, the open ends of which face up stream. These larval inhabitants feed on the minute plants and animals swept into the nets. Free living caddis larvae Rhyachophila can roam over the stones.
Algae and water moss also are attached permanently to the substrate. Water moss and heavily branched filamentous algae are held to rocks by strong hold fasts and are aligned with the current. Algae growth in streams often exhibit zonation on rocks. The growth is influenced by both depth and current.
In addition, the algae are covered with a slippery gelatinous coating. Other algae grow in spherical or cushion like colonies with smooth, gelatinous surfaces. Some species are reduced to simplified plate like forms which grow in closely depressed sheets that follow the contours of the rocks.
In limestone water, some algae such as Phormidium and Androinella secrete calcium carbonate to cover the entire algal growth with a crust. These may be overgrown at certain seasons by such algae as cladophera.
Other organisms that live in the rapid water are attached by sticky undersurfaces such as snails and planarians which help them remain stationary in the current. The few cilliates that live in the stream differ very little from those of still water and exhibit no special adaptations for the current. Instead, they merely live in the bottom areas where currents are close to zero or negligible.
In spite of these adaptations, many bottom organisms tend to drift downstream to form a sort of traveling benthos. This is a normal process in streams, even in the absence of high water and abnormal currents. This drift consists mainly of organisms that are free ranging, such as may fly larvae, Baetis and the scud Gamarus.
The drift of stream insects is not a haphazard incident. It is relatively constant and exhibits diurnal and seasonal variations through the year. It is usually greatest at night, especially very soon after sunset and lowest during the day.
Drift is also influenced by temperature, different species reacting differently to different seasons. Drift, however great, unless catastrophic rarely depletes the stream population of drift organisms. Drift is so characteristic of streams that a mean rate of drift can serve as an index of the production rate of the stream.