What sort of strange diseases lurk in the shadowy waters of Chicago’s rivers?

You’d think that’s something we’d already know.

But it turns out that scientists don’t know how risky recreating on the Chicago River is.

But now that the river is cleaner than decades past – or at least less dirty – more than ten thousand people each year float it, paddle it – even wade in it.

So the Metropolitan Water Reclamation District has teamed up with the University of Illinois-Chicago on a big epidemiological study.

It’s called the Chicago Health, Environmental Exposure, and Recreation Study, or CHEERS.

Researchers hope to recruit 2,000 river users and others, and track them for signs like skin disease, eye irritation and intestinal issues.

The effort began last month with stations like this one – set up under a tent at the Skokie rowing center, on the north branch of the Chicago River.

This weekend CHEERS recruiters will be out in force.

They hope to scoop up 200 new volunteer subjects at Sunday’s Flatwater Classic.

The race, sponsored by the Friends of the Chicago River, gets underway at 10 a.m. in Chicago’s Clark Park.