Guest column: Take a good look at Indiana's streams
This guest column was written by Michael Litwin, a member of the Bloomington Environmental Commission.
Spring is here and people are spending time outdoors for a safe activity during the epidemic, so I’m providing some nature information.
When people look at a stream, they probably assume that its shape is somewhat random, constantly changing with floods and other temporary disturbances. In fact, the shape of a stream is predictable, primarily determined by characteristics of its watershed, such as the amount of rainfall, the steepness of its landscape and the soil type. As the stream corridor progresses downstream, those characteristics change, and so will the shape of the stream channel. If a stream scientist has all of that physical information along with some stream flow measurements he or she can predict the shape of a stream fairly accurately. The name of stream science is a mouthful: fluvial geomorphology.
Most rivers and streams in the Midwest are connected to the water table during at least part of the year, providing a “base flow” even without recent rain. Stable waterways have predictable patterns of features, such as meanders (channel bends or loops), pools, riffles (shallow, fast flowing rocky areas) and runs or glides (moderate/fast flowing stretches between pools and riffles). Even after a large flood the river channel returns to an approximation of its predictable shape, which typically reflects the stream’s high flow that recurs every 1 1/2 to 2 years on average.
Fish and other aquatic animals have adapted to these channel characteristics. The same physical features: riffles, runs and pools, are also habitat for fish. Some species use several habitats during their life cycle. Altogether, there are about 200 species of fish in Indiana, some of which occupy only specific habitats in clean streams and are seldom observed without specialized collecting gear.
If a stream becomes unstable, habitat features can become degraded or disappear completely. This is where storm water flow enters the picture. If the physical conditions that determine a stream’s shape change, the shape will also change to adapt to the new conditions. In nature these sorts of changes occur over thousands of years, but in areas developed by people they can change radically in a few years.
In a typical undeveloped landscape, over 50% of rainfall sinks into the soil and slowly makes its way to the water table. About 10% runs off overland until it reaches the nearest waterway. At the peak of storm runoff the stream reaches its “peak flow,” and its shape is in equilibrium with that peak flow.
When an area becomes heavily urbanized it contains a progressively larger amount of impermeable surface, chiefly concrete and asphalt. Only 10% or less of rainfall reaches the water table in heavily developed areas, and over 50% runs off to the stream, overland and through storm sewers. A much higher peak flow during rain events means greatly increased water flow in the stream, which causes a stream channel to become unstable, cutting the channel deeper, with erosion and continuous alteration of the channel shape until it reaches a new equilibrium.
A second consequence of the increased runoff and reduced soil infiltration during a rainfall event is that the water table is deprived of some of its water source. The water table becomes lower, and as a result, intermittent streams dry up sooner in the summer. This reduces the amount of time that the stream can support a fish community, and also reducing its ability to dilute pollutants.
Next time you look at a stream, notice whether the channel appears to have a natural or altered shape. Does it have meanders and riffles, or is it steep, straighter and eroding?