From the air, the edges of a stream appear as bright green tracks furrowed into the forest floor. That's because grasses, shrubs, and trees flourish in the damp, fertile soil of the banks and floodplains that border the stream.
This is the riparian zone, a protective margin of vegetation that keeps the water clear and cool for the creatures of the stream. It is also important to almost all of our forest animals, which come not only to drink, but to find food, shelter, and hiding places. In summer, shade from the vegetation along the water's edge moderates temperature, humidity, and light. In winter, thickets of trees and vines buffer the area from harsh winds. This provides a protected habitat for many woodland animals.
Alder, maples, willows, and many other plants stabilize the stream banks and prevent erosion from storms and snow melt. Their leaves intercept much of the rain before it reaches the ground, returning it to the atmosphere as water vapor. Storm water runoff carrying potential natural pollutants such as silt, nitrogen, and phosphorus, is absorbed and used by the plants before it enters the stream. Soils also store excess water underground, releasing it slowly over time to prevent downstream flooding. Pesticides and other human-generated pollutants may be retained by the soils long enough for bacteria to break them down into harmless compounds.
Birds build their nests in trees along the bank from mud and twigs gathered along the shore. They feed on the seeds and berries of streamside shrubs or on aquatic insects. Deer and moose come to the stream to drink. In winter, they travel its frozen corridors to their feeding grounds and yard in its sheltered valley. Weasels, mink, skunks, and mice scurry beneath the shade of the streamside forest to avoid the notice of owls, hawks, and other sharp-eyed predators. Night-prowling raccoons leave tiny leprechaun footprints in the muddy banks as they hunt for crayfish and freshwater mussels.
Flying insects that drop into the water from overhanging branches are quickly devoured by predatory fish like trout and salmon (Fishing lures are designed to imitate this riparian food supply that splashes into the stream from above.) Ultimately, the largest fish in a river is dependent on leaf litter from the riparian zone. An alder leaf that is swept into a stream in Eustis is broken down and recycled as it moves downstream. It feeds an insect that soon is gobbled by a small fish which eventually is eaten by a trout that in turn nibbles at the hook of a fly fisherman on the lower Kennebec.
The riparian zone: where human action alters the course of nature
Here in the riparian zone conflicts often arise because private property, the stream bank, abuts a public resource, the water. What we do on our own land affects what happens far beyond our property line. Even a narrow gash in the riparian zone, made by a stream crossing for cows, a log skidder, or an all-terrain vehicle, opens the stream to runoff of silt and pollutants from the land. Silt clouds the water, scratches the delicate gills of fishes, and smothers aquatic insects and fish eggs. A build-up of silt makes the river bed more shallow and prone to overflow its banks in a heavy storm. Phosphorus, nitrogen, and other nutrients washed in with the soil cause noxious algae blooms.
Where trees and shrubs have been cleared entirely from the water's edge, erosion turns the stream mud brown. Without a shady canopy, the water becomes too warm for fish like salmon and trout. If the food supply of leaf litter and falling insects stops, many aquatic animals either starve or move to a more favorable environment. When we breach this wall of vegetation we begin a chain of destruction that threatens habitats, food chains, and the quality of the water far downstream.