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
LIFE ON THE BOTTOM OF A STREAM
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
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
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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Update: June 2012