Important though it is, there is more to a stream than water. The stream is enriched by a wide variety of animals and plants that live along or within its course. The kinds of animals and plants found in a stream change from its upper reaches to the broader stretches downstream, yet its headwaters supply water and food for the entire stream system in what is called the river continuum.
The base of the food chain in the stream, as on land, is plants. But unlike forests or lakes, the stream doesn't rely directly on the energy of the sun (through photosynthesis) to generate its own plant life. Small streams are often too shady or too turbulent to support much microscopic algae or to allow larger plants to take root. So where do the plants of the stream come from? They fall in.
Plant litter tumbles in from the edge or from above, essential food and nutrients for the animals and plants of the water. Some animals such as mayfly and stonefly nymphs nibble directly on the fallen leaves. They are known as shredders. Bacteria and fungi also attack the plant material. Branches and leaves that would have been undigestible in their original state are converted to minute, mushy particles of detritus, rather like baby food, for other aquatic insect larvae, crayfish, and snails. The aquatic stages of caddisflies, mayflies, stoneflies, and blackflies then become food for fish, salamanders, turtles, and other wildlife throughout the stream.
It's not easy to maintain your grip in a frigid, fast-moving mountain stream. Performing everyday functions like eating, moving, or holding on as the water cascades over rocks and waterfalls is like a mountain climber maneuvering across a rock face in a gale-force wind. The creatures of the headwaters have evolved their own versions of grappling hooks, rappelling ropes, and crampons.
Blackfly larvae (those woodland terrorists when they reach adulthood) anchor their bottoms to rocks and capture plankton through feathery filters that sway in the current. If they are torn from their roost, they lasso onto another rock with silken threads before they can be swept away. Other insects, such as stonefly and mayfly nymphs, have evolved streamlined profiles which they secure to rocks by hooks on their feet.
Caddisfly larvae make stony caves or stick shelters to protect their soft bodies by cementing together grains of sand or pieces of vegetation. They partially emerge from their handcrafted huts to grab meals of algae and dead leaves. Because these insects thrive in clean, clear water, their presence is used to gauge the water quality of streams.
Though living in strong currents is a challenge, it also has its benefits. The churning water mixes oxygen from top to bottom and distributes nutrients from soil runoff and decomposing plants and animals. The headwaters of a stream provide a nourishing habitat for any creature that can buck the current or find a way to avoid it. Here are the spawning grounds for many fish and insects that later wash downstream to supply the entire stream system. With luck, some return to the headwaters as adults to lay their eggs and start the process all over again.
Young brook trout and Atlantic salmon prefer cold, oxygen-rich streams where they feed on immature insects below and adults above the water's surface. Sheltered pools, back eddies, and fallen logs let them rest on their downstream journey to the sea or on their return to the headwaters of their birth. Strong, streamlined bodies and powerful tails help them battle the current to reach their spawning grounds upstream.
As the slope decreases, so does the speed of the current. Rather than cutting downward, the water spreads out in a broad, meandering course. In these calmer waters you can find a whole community dependent on the interface between air and water, the surface film created by the strong attraction of water molecules called surface tension. Water striders skim across the film like skaters at an ice rink. Their needlelike mouthparts drain the body fluids of other surface-dwelling insects. In quiet backwaters, mosquito larvae hang upsidedown, suspended by the surface tension. They suck oxygen from the air through snorkels in their tails. Backswimmers and whirligig beetles grab bubbles of air from the surface and carry them around like miniature SCUBA tanks.
Worms, crayfish, and clams burrow into the mud deposited by the slower-moving current. With fearsome hinged jaws that jut out to capture prey, dragonfly and damselfly nymphs snatch other insects and even small fishes.
Fish like bass and perch (which require less oxygen and tolerate warm water better than trout and salmon) live quite comfortably in the slower current of the river. But should the oxygen level of the water drop too much, only pollution-tolerant species such as sludge worms can survive.
In the broader stretches of the river, plants creep out from the shoreline into shallow water to create freshwater wetlands: wide, low areas that flood in spring and in heavy rains. These wetlands filter out sediments, nutrients, and pollutants, and reduce upland flooding. They also provide shallow pools where pickerel, perch, and salamanders can lay their eggs beyond the reach of deeper water predators. Wetland plants slow the current and protect the river bed from erosion while they create cover for frogs, turtles, waterfowl, and other animals.
It is important to protect all the plant and animal life forms in a stream, because these diverse and abundant life forms create a stable community which is resistant to both human and naturally induced stresses. This diversity of life forms also contributes to a functional community, one which moves material and energy up the food chain efficiently, supporting many layers of life.