How long ago it rained

Though the river may be improving, organized swimming hasn’t been authorized in more than a century.

Timothy Hoellein, an aquatic ecologist at Loyola University Chicago who studies urban ecosystems, said that’s because heavy rains and a large urban population can contribute to dangerous conditions.

Chicago has a combined sewer system, which means stormwater runoff and sewage flow through the same pipes. That waste is decontaminated at a wastewater treatment plant, which removes disease-causing organisms before they enter the river.

But, intense downpours can overwhelm the system and cause overflows that send untreated runoff and human waste directly into the river.

According to the Metropolitan Water Reclamation District, sewage was discharged into the Chicago River because of heavy rains a handful of times last summer.

So much rain fell over the July 1 weekend that waste and runoff poured out of nearly a dozen overflow pipes across Cook County, from Evanston to Westchester, many for hours at a time. MWRD had to open the locks near Navy Pier to relieve pressure on the system, allowing more than 1.1 billion gallons of waste to flow into Lake Michigan.

“There’s been really, really major changes in how we manage our drinking water, and our wastewater and our environmental footprint as it relates to the river,” Hoellein said. “But when there’s been one of these combined sewer overflows, there’s more potential for disease-causing organisms and microbes in the water.”

As long as everything in the river has gone through a wastewater treatment plant, Hoellein added, it’s safe to swim.

“Whether I’d get in depends on how long ago it rained very hard,” Hollein said. “A couple of days or a week or so after, I think it would probably would be OK. But if you’re talking like right after a big rainfall or right after a plant overflow, I would not (get in).”

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