Slow Streams and Rivers
As steam and river current slows, a noticeable change takes place in streams. Silt and decaying organic matter begin accumulating on the bottom. Faunal organisms are able to move about to obtain their food and plankton populations may develop. Fine detritus particles from upstream become the main source of energy.
In many ways, the composition and configuration of the stream community approaches that of standing water. These waters approach that of lakes or ponds even to including zonation along the river margin. The high water temperature, week current and abundant decaying matter promotes the growth of protozoan and other plankton populations.
Even though scarce in fast water, plankton increase in numbers and species in slow water. Since rivers have no typical plankton of their own, those found there originated mainly from backwaters and lakes. In general, plankton populations in rivers are not nearly as dense as those in lakes.
With increasing temperatures, decreasing current and accumulating bottom silt, organisms of fast water are gradually replaced by organisms adapted to these slower water conditions. With current at a minimum, many resident fish lack the strong lateral muscles typical of the trout. Instead, they have compressed bodies that permit them to move with ease through the masses of aquatic plants. Brook trout and sculpin give way to the small mouth bass and rock bass, the dace to shiners and darters. Only in occasional stretches of fast water in the center of the stream are remnants of the headwater stream organisms found.
As the volume of streamflow increases, and as the current becomes even slower, the silt deposits get heavier and the detritus feeders increase in numbers. Tube dwelling annelids and midges are common as well as the bottom feeding catfish, suckers and the carp. The insects, backswimmers, waterboatmen and diving beetles also inhabit the sluggish stretches and backwaters of rivers. Where water conditions are suitable, musky, pike and turtles are also common.
Rooted aquatics begin to appear when current slows enough for plants to establish and grow. The emergent vegetation grows along the river banks and duck weeds are found floating on the still water surface.