Forest Preserve District of Cook County, Illinois
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Life on the Bottom of a Stream
Nature Bulletin No. 690   October 20, 1962
Forest Preserve District of Cook County
Seymour Simon, President
David H. Thompson, Senior Naturalist

A stream conceals a teeming world of bottom-dwelling animals that are the food supply for all stream fish and a source of live bait for catching them. Raccoons, mink, muskrats, ducks, shore birds, turtles and frogs hunt here for mussels, snails, crayfish and aquatic insects. These insects, after passing their young stages on the stream bottom, emerge as swarms of flying adults devoured by dozens of kinds of song birds. These, too, are the insects that fly fishermen imitate in making their artificial lures.

Streams of all sizes have about the same kinds of bottom animals, whether a brook small enough to be stepped across or the mile-wide Mississippi. The greatest differences are found when the populations from different types of bottom are compared -- rock, gravel, sand and mud. These main types result from the sorting action of the water, especially during floods. Rock bottom is found in the fastest water because all smaller materials are swept downstream. As the current becomes slower the gravel, then the sand, and finally the mud, settle out.

A flat rock with water swirling around it on the riffle of a clean stream hides dozens of small aquatic animals. If the rock is lifted, crayfish and perhaps a small fish are glimpsed as they scurry into other hiding places. On its underside, flat-bodied mayfly nymphs with tufts of gills on the sides skitter over the wet surface. Also, here are slender stonefly nymphs with two caudal filaments. Caddisfly larvae, which weave tiny nets to catch their food, are seen and sometimes a strange species that lives in a coiled tube made of sand grains glued together. Both air- breathing and gill-breathing snails may be present, as well as creeping adult beetles. Abroad, rubbery leech, clinging with suckers fore and aft, may be hovering over a blob of bright yellow eggs. A long slender leech glues brown seed-like egg cases to the rock. With luck, you may find a hellgrammite, the big ferocious-looking young of the Dobson fly and a favorite bait for catching game fish.

Gravel bottom usually supports more pounds of animal life per acre than any other part of a stream. When a square foot of it is dredged up, picked over carefully, rinsed and strained, it commonly yields a wriggling mass weighing an ounce and made up of 20 or 30 species. Also common here are the little fingernail clams, or "duck shells", whose pinhead-sized young are born fully formed. The most striking creatures on the gravel bottom of creeks and rivers are the large, thick- shelled mussels from which pearl buttons are made.

The sand bottom of a stream, like a sand area on land, often is an almost lifeless desert. Perhaps sand shifts about too frequently or, perhaps it provides little food and shelter for freshwater animals.

The slow addition of sediment makes mud bottom a rich underwater soil. Blood worms, tiny relatives of earthworms, have mud tubes into which they retreat. The midges that swarm at our windows at night come from mud-dwelling younger stages. The heaps of "cisco flies ' that pile up under street lights in river-front towns come from the large mayfly nymphs that burrow in mud. Mollusks are represented by duck shells and kinds of mussels and snails not found in swifter water. Leeches squirm and dragonfly nymphs lumber over the oozy bottom.

When a clean stream becomes polluted with sewage, most of the bottom animals die. Mayflies, stoneflies and caddisflies are most sensitive and disappear first. With more and more pollution, others drop out one by one. At last the bottom is covered with nothing but a waving mat of sludge worms like the thick pile on a rug. If the pollution is stopped, the animal life slowly comes back.

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