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according to those same inexorable laws—downhill in its search for the sea. As we watch, the rip-
ples die away, leaving as little mark as the usual human lifespan creates in the waters of the world,
then disappear as if they had never been. Now the river water is as before, still. At the pool's edge,
we peer down through the depth to the very bottom—the substrate.
We determine that the pool bottom is not flat or smooth but instead is pitted and mounded occa-
sionally with discontinuities. Gravel mounds alongside small corresponding indentations—small,
shallow pits—make it apparent to us that gravel was removed from the indentations and piled into
slightly higher mounds. From our topside position, as we look down through the cool, quiescent
liquid, the exact height of the mounds and the depth of the indentations are difficult for us to judge;
our vision is distorted through several feet of water.
However, we can detect near the low gravel mounds (where female salmon buried their eggs
and where their young grow until they are old enough to fend for themselves), and actually through
the gravel mounds, movement—water flow—an upwelling of groundwater. This water movement
explains our ability to see the variegated color of pebbles. The mud and silt that would normally
cover these pebbles have been washed away by the water's subtle, inescapable movement. Obviously,
in the depths, our still water is not as still as it first appeared.
The slow, steady, inexorable flow of water in and out of the pool, along with the upflowing of
groundwater through the pool's substrate and through the salmon redds (nests) is only a small part
of the activities occurring within the pool, including the air above it, the vegetation surrounding it,
and the damp bank and sandbar forming its sides.
Let's get back to the pool itself. If we could look at a cross-sectional slice of the pool, at the water
column, the surface of the pool may carry those animals that can literally walk on water. The body
of the pool may carry rotifers and protozoa and bacteria—tiny microscopic animals—as well as
many fish. Fish will also inhabit hidden areas beneath large rocks and ledges, to escape predators.
Going down further in the water column, we come to the pool bed. This is the benthic zone, and cer-
tainly the greatest number of creatures live here, including larvae and nymphs of all sorts, worms,
leeches, flatworms, clams, crayfish, dace, brook lampreys, sculpins, suckers, and water mites.
We need to go down even farther, down into the pool bed, to see the whole story. How far we
have to go and what lives here, beneath the water, depends on whether it is a gravelly bed or a
silty or muddy one. Gravel will allow water, with its oxygen and food, to reach organisms that live
underneath the pool. Many of the organisms that are found in the benthic zone may also be found
underneath, in the hyporheic zone. But to see the rest of the story we need to look at the pool's
outlet, where its flow enters the main river. This is a riffle area—a shallow place where water runs
fast and is disturbed by rocks. Only organisms that cling very well, such as net-winged midges,
caddisflies, stoneflies, some mayflies, dace, and sculpins can spend much time here, and the plant
life is restricted to diatoms and small algae. Riffles are a good place for mayflies, stoneflies, and
caddisflies to live because they offer plenty of gravel to hide in.
Earlier, we struggled to find the right words to describe the river pool. Eventually, we settled on
still waters . We did this because of our initial impression, and because of our lack of understanding
and lack of knowledge. Even knowing what we know now, we might still describe the river pool as
still waters. However, in reality, we must call the pool what it really is—a dynamic habitat. Each
river pool has its own biological community, the members interwoven with each other in complex
fashion, all depending on each other. Thus, our river pool habitat is part of a complex, dynamic eco-
system. On reflection, we realize, moreover, that anything dynamic certainly cannot be accurately
characterized as still—including our river pool (Spellman and Drinan, 2001).
20.1 STILL WATER SYSTEMS
Freshwater systems may be conveniently considered in two classes: running water and still or stand-
ing water systems. There is no sharp distinction between the two classes. Lakes are defined as
basins filled with water with no immediate means of flowing to the sea, containing relatively still
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