Short essay on Rivers and Streams: Running Water

The continuous one-way flow of running waters distinguishes them from lakes. Indeed, many of the special characteristics of these waters are related to their turbulence.

Because currents constantly bring new water into contact with a given organism, running water is effectively richer than still water in which a static, “tired” layer surrounds each individual. And to be sure, turbulence mixes air into running water and thus usually ensures a plentiful supply of oxygen.

The Food Web of an Open System

Rivers and streams inevitably reflect the-land they drain. Stream water arises mainly from groundwater, and surface runoff provides most of the remainder. Water from both sources carries mineral nutrients and human additives, such as pesticides.

But most of the organic matter that supports the food chains of running waters also washes in from the surrounding land. Or simply falls in.

Probably tree leaves supply more energy and more carbon than any other single source. Additional sources of carbon and energy are adult insects that flounder into the water, earthworms, and other small soil animals. Except in quiet streams that closely resemble lakes or ponds, there are few phytoplanktons in rivers.

Thus running waters are unusual ecosystems in that primary production accounts for but a small portion of the energy needed. Most nutrients enter rivers and streams from the outside.

Adaptation of Currents: Don’ts Get Carried Away

Only careful strategies prevent the current from carrying stream organisms to different and less suitable habitats. For example, diatoms of such habitats have stalks.

Larvae of Black flies and many other insects anchor themselves with holdfasts. Yet other insects spin sticky threads about themselves or burrow into the mud. Many stream animals are streamlined. They have compact and flattened bodies and usually live under stones, where the current is reduced and where they are less visible to predators.

Fish can expend a great deal of energy swimming against the current-so they don’t. Fish survive by finding shelter. As anyone who fishes knows, the best place to catch trout is in a deep bottom hole or along the downstream side of large rocks or other obstacles. Generally, the larger the shelter, the larger the fish.

The adaptations of stream organisms truly illustrate the selective forces of this environment. But of course, every environment is selective.