More than a century of ecological research on the Illinois River highlights the importance of clean water, from Peoria to Chicago.

Toward the end of the 19th century, one of America’s greatest ecologists set up shop on the banks of the Illinois River. Stephen Forbes wanted to know how the Illinois River worked: how it was able to produce such large quantities of fish and fowl. He made great progress in understanding how the fertility of the floodplain helped fuel such unusually high levels of fish and waterfowl. However, Forbes’ research soon took a different turn as he began to record the impacts of the Chicago Sanitary and Ship Canal.

When it opened in 1900, the canal was a tremendous feat of engineering—and an answer to a pressing problem. Chicago’s wastewater was being discharged into Lake Michigan, which was also the source of its drinking water. The canal reversed the flow of a river that emptied into Lake Michigan and excavated a new path to the headwaters of the Illinois River. On one hand, the canal was a great success: it stopped sewage from fouling the source of Chicago’s water supply. On the other hand, it had simply rerouted the wastewater, pushing its effects elsewhere.

In the decades after the canal opened, Forbes and his colleagues at the Illinois Natural History Survey studied how this influx of wastewater was changing the Illinois River, measuring water quality between Peoria and the canal.

Dead Zones & Lack of Diversity
In the summer, it was common for oxygen levels in the water to be completely depleted. Dead zones, or regions of water without enough oxygen, are most often discussed with regard to the Gulf of Mexico. Essentially the same problem that currently occurs in the Gulf happened to the Illinois River—excessive nutrient inputs caused oxygen levels to plummet. Not surprisingly, such low oxygen levels affected the fish in the river. While commercial fishing continued in the lower portions of the Illinois River, many of the most desirable fish were virtually absent upstream.

In the 1950s, scientists began to systematically sample the fish in the Illinois River. Species considered to be sport fish, such as bass and sunfish, were few and far between. In some areas, more than 95 percent of the fish sampled were common carp or goldfish, two pollution-tolerant, invasive species. In almost every year since, the Illinois Natural History Survey has continued to sample the fish, maintaining the same procedures each year.

A Dramatic Recovery
In recent decades, this trend has reversed. Since the 1990s, once-absent populations of sport fish have made dramatic recoveries, particularly in the most upstream areas. In addition to producing a large number of fish, the river now sustains high levels of biodiversity.

“One thing that impresses me is the diversity of sunfish species we catch,” says Jason DeBoer, a fish biologist for the Illinois Natural History Survey. Familiar species, such as bluegill and green sunfish, occur alongside other species, like longear sunfish, pumpkinseed sunfish and orangespotted sunfish. DeBoer also notes the population of big fish is doing well. “The upper Illinois River is one of the few places we catch big largemouth bass. The population seems robust.”

So, what caused these positive changes? The evidence suggests improving water quality led to the turnaround. As the treatment of wastewater improved, so did populations of fish like bluegill and smallmouth bass.

New Competition
Today, the abundance of sport fish in the upper Illinois River can be considered a great success story, showing the clear benefits of clean water. However, their continued health faces a new challenge from invasive Asian carp. Although Asian carp don’t eat bass or sunfish, they do compete with the same resources these species need when they are young. Juvenile sport fish have a diet that is similar to that of the adult Asian carp.

Asian carp are highly efficient at filtering small food items from the water. When the number of Asian carp becomes high enough, it’s likely these invasive species will have an impact on the sport fish. No matter how good conditions are for adult sport fish, a lack of food resources for juvenile sport fish could act as a bottleneck. For this reason, the Illinois Department of Natural Resources has been removing Asian carp from the upper Illinois River. These removals can reduce the threat of invasion in Lake Michigan, and they will also probably reduce competition with native sport fish.

A Brighter Outlook
Forbes and his colleagues would probably be pleased to see how much progress has been made on the Illinois River. The return of so many species, including some of the most recreationally popular fish, illustrates the importance of clean water and the value of collecting biological information. It’s also a reminder that many environmental problems, from poor water quality to invasive species, can often lead to positive outcomes.

It’s not often that a new century begins with a brighter environmental outlook than the one that preceded it, but there are many reasons to have high hopes for the future of the Illinois River. iBi

Daniel Gibson-Reinemer is a postdoctoral research associate at the Illinois Natural History Survey.