How does this event positively benefit the river? How can we build into the process and the event a benefit (instead of a contamination).
This is a bit out there, but what is to say that the smaller floats actually improve the river. Low cost floating bio-remediation wetlands could be built within a school educational component about the event or about the river.
More Personal projects could use materials that will benefit the river, such as walnut shells, or barley straw to manage algae, or chelating fish food to improve the animal health (A-la Jeremijenko)
River Remediation is always a large scale investment. why can't it be a low cost, public event?
This would be a hugely complicating aspect of the event, and a lot of research would need to be done as to the effects of the event, but the potential is there to help the Chicago River while celebrating the ideas of Renewal and the Great Chicago Fire.