The source of Tyler Creek is in farmland in Rutland Township in Kane County. It flows east and south through new suburban developments and golf courses, an Elgin city park, a Kane County forest preserve, and the grounds of Judson College before joining the Fox River.

Tyler Creek has managed to retain a quality not common in streams in northeastern Illinois: on a sunny summer day it has the look and smell of a pristine stream. In the forest preserve its gradient is steep. The current flows so fast that silt and sand have both been scoured from the bottom. The creek is cobble bottomed, covered with rocks as big as softballs. Lift one of them and you might see a sign of Tyler Creek’s quality. Sticking to the bottom or sides of the rock might be a little cluster of angular pebbles–the home of a caddis fly larvae, a refuge for an insect that lives only where the water is largely free of pollution.

Caddis fly larvae live in streams for as long as a year before pupating and developing into adults. The adult is short-lived and rarely eats. It reproduces and dies. Females drop eggs onto the water or attach them to branches overhanging the water. When they hatch, the larvae head for the bottom.

The shelters the larvae build vary from species to species. Some species build movable shelters that they drag about the bottom. Others build stationary tubes like the ones I saw. Their shelters may be constructed of plant material, such as twigs or rolled leaves, or they may be built of sand or gravel. In the fast-moving waters of Tyler Creek, fairly good size pebbles are probably the only materials available.

Aquatic biologists have developed a classification system for the macroinvertebrates of our streams and rivers. (“Macro” here means that the creatures can be seen with the naked eye.) The system divides the typical macroinvertebrate fauna into four broad categories based on pollution tolerance.

The least pollution-tolerant class, group I, includes stone fly naiads, and the larvae of alderflies, dobsonflies, and snipe flies. Immature stone flies are called naiads because they do not undergo a complete metamorphosis to become adults. They look very much like adult insects, with six legs, two long antennae, and two long tails–cerci–projecting from the rear of their abdomens. Alderfly and dobsonfly larvae look like caterpillars, and snipe fly larvae are very wormlike.

The key factor limiting these animals to the cleanest water seems to be oxygen levels. Dump sewage or animal waste into a stream and the bacteria of decay go to work on the organic compounds in the waste–and their respiration uses up oxygen. Fertilizers washed into creeks stimulate algae blooms, and when the algae die the bacteria go to work on them.

Stone flies live as aquatic naiads for as long as three years before emerging from the water as adults. Some are predators, but most live on algae and other plants. Most adults cannot feed; their mouthparts are vestigial.

Our caddis flies fall into group II, which includes dragonfly and mayfly naiads, assorted beetles, crayfish, clams, and mussels. As pollution increases the group I animals become less and less common and eventually disappear. Mayflies are among the first to take advantage of the increased nutrients. They eat algae and small animals, and they can live in the water for as long as four years. The emergence of adult mayflies has to be precisely timed. They fly up from the water in clouds, and they mate immediately. Some species emerge in the evening and are dead by morning. They cannot eat as adults.

Dragonfly naiads can tolerate moderate levels of pollution, but as predators–they feed on smaller insects and even on small fish–they are at the top of the food chain and can be harmed by metals and persistent organic chemicals that have entered the bodies of their prey. Crayfish, as scavengers, face similar problems. Some of our local streams have become attractive nuisances. Cleanup of conventional pollutants such as sewage has progressed to the point that dragonflies and crayfish can return, but now these creatures are helping metals and persistent organics enter the food chain.

Group III, animals fairly tolerant of high pollution and low oxygen levels, includes the tiny crustaceans called scuds, right-handed snails, and the larvae of midges. Scuds are usually less than a half inch long and bear a family resemblance to shrimp. They are scavengers and a major food source for many larger animals, including fish.

You may be asking yourself what a right-handed snail could be, in light of the fact that snails don’t have hands, but if you hold a snail with the pointed end of the shell facing up and the opening at the bottom of the shell facing toward you, the opening will be either on your left or on your right. If it is on your right you are looking at a snail that breathes with gills. If the opening is on your left the snail breathes through a lung.

Some midge larvae are called bloodworms. They are bright red, a color they get because their blood is extremely rich in hemoglobin. Hemoglobin is the substance that absorbs oxygen and allows the blood to transport it around the body. Very high hemoglobin levels are a useful adaptation for animals that live in environments that are low in oxygen.

Sludge worms, aquatic relatives of earthworms, are loaded with hemoglobin. They are such a bright red they practically glow in the dark. They live with their heads buried in the bottom and their tails waving in the water looking to catch any stray oxygen molecules that happen by. These are the most pollution-tolerant animals in this part of the world–and probably in any part of the world.

Like earthworms, they feed by passing whatever they can swallow through their bodies, extracting any organic matter that happens to be present. A few years ago the bottom of Indiana Harbor was easily the most polluted place in the Great Lakes. It was loaded with sewage, heavy metals, and assorted toxic chemicals. Repeated sampling of the bottom mud showed that it was almost entirely without life. Only two living organisms were found, and they were both sludge worms.

Carp eat sludge worms. If you eat carp, you want to be very careful about where it came from. Carp caught in polluted waters are just loaded with toxics.

You can tell a lot about the water quality of a stream by studying the macroinvertebrates that live in it. You need extensive sampling by various methods and in various kinds of waters–riffles and pools, for example–to be able to draw any conclusions, but with thorough sampling you can actually put a number on a stream that serves as an index of water quality.

I expect that Tyler Creek is high quality. The Chicago River is not, though it has improved substantially. There are certainly many more species and many more individual organisms than there used to be, but so far they belong pretty much to groups III and IV.

The extensive sampling is needed because while the presence of stone flies indicates high quality, the presence of sludge worms does not necessarily indicate low quality. Sludge worms can be present in high quality streams, though they will be there in much lower numbers, and they will be one of many species in the stream.

The macroinvertebrate populations of streams and rivers are perfect examples of the effects pollution has on biodiversity. Thoroughly sample a stream; sample the headwaters where trees shade the water and keep the temperature low. Sample downstream where some of the water is open to the sun and temperatures fluctuate daily. Sample riffles and pools, slow water and fast, gravel bottoms and mud bottoms. Sample all this and you might find every insect, crustacean, mollusk, gastropod, and oligochaete mentioned in this article. Start adding fertilizer runoff, sewage and animal wastes, and industrial pollution, and you can see the diversity vanish. Reduce the stream to the level of the worst of our rivers and you may have nothing left but sludge worms. There may be millions of those, but where there was once an ecosystem, there is now only a single species. The ecological niches that used to be filled by stone flies and caddis flies and dragonfly naiads are empty. And the larger animals that once fed on these are gone too.