A work crew will punch a hole through the center of the Hofmann Dam on June 18, and water will begin to recede into the center channel of the Des Plaines River after the Riverside village board refused to halt the work on Monday.

After three hours of comment from residents both for and against the project at a special meeting of the village board, trustees gave their blessing to the dam-notching, based partially on a written assurance from the Illinois Department of Natural Resources (IDNR) that the agency, along with the U.S. Army Corps of Engineers, would “mitigate” any “unintended impacts” resulting from the operation.

Read all the Hofmann dam documents here.

Approximately 50 people crammed into Room 4 at Riverside Township Hall and 17 people spoke on the subject of the dam, with evenly split views regarding its removal.

“We have a commitment by IDNR to mitigate whatever damage is done,” said Village President Michael Gorman. “They don’t agree there’s going to be any damage. We have exerted the political will to the extent we could with this project.”

But residents who live along Maplewood Road remained unconvinced. They complained that a flurry of conflicting numbers have been presented to residents in the past week and that questions still remain about the actual impact the dam-notching will have on water levels.

Specifically, homeowner Jeff Miller questioned apparently conflicting conclusions drawn by the Army Corps of Engineers and the USGS regarding the estimated river width following the dam removal in the area of Maplewood Road.

Those conclusions – charts and maps generated by the U.S. Army Corps of Engineers and a PowerPoint water flow analysis developed by the U.S. Geological Survey – were unveiled on June 7 at a special meeting held at P.J. Klem’s in Lyons.

The Corps estimated that the width of the river at low flow would shrink by 50 feet and would leave 18 feet of mud flats adjacent to their properties. The USGS, meanwhile, estimated that the width in a location near the one identified by the Corps would be narrower by 22 to 36 feet.

“In my seemingly boundless naivete, I had assumed that the numbers presented by the USGS and the ACE would agree, at least approximately,” wrote Miller on Tuesday morning to Arlan Juhl, director of the IDNR’s Office of Water Resources. “I wasn’t aware that there might be large differences between them, even as of last Thursday.”

Village trustees on Monday also struggled to make sense of all the new data, which included everything from new information regarding the impact of Salt Creek flow on the Des Plaines to the impact of a structure near 31st Street that diverts Salt Creek water into the Des Plaines and the amount of water that waste treatment plants contribute to the flows.

But trustees were convinced to move ahead after Juhl promised to mitigate any fallout from the project. Juhl’s promise came in a letter just an hour prior to Monday’s special board meeting. His letter was a response to concerns voiced earlier Monday in a letter to the IDNR by Maplewood Road residents about the impact of the project to their properties.

“We believe that all impacts of the Hofmann Dam removal or changes in water conditions have been identified and mitigated,” Juhl wrote. “Any unidentified impacts that may surface later would be the responsibility of the IDNR and the Corps of Engineers to mitigate.”

Gorman stated that the IDNR has agreed to expand its work area for replanting the riverbank from 2,000 feet behind the dam all the way to 31st Street and that the IDNR would mitigate any lack of vegetation for up to three years following the completion of the dam removal.

Between Monday and June 18, when the dam is removed, the village attorney, Village Manager Peter Scalera and Maplewood Road residents will be working with the IDNR to hammer out language regarding just what the IDNR and Army Corps will agree to “mitigate” if there’s damage to private property as a result of the dam removal.

“Nailing it down is not going to be an issue,” said Gorman.

The village called for an emergency halt to the dam-notching in April after it was revealed that river flow data used to estimate water levels after the dam’s removal was faulty and that another, more accurate, set of numbers had not been publicly disclosed.

New data generated in September 2011 by the IDNR’s Office of Water Resources suggested that removal of the dam would have a greater impact on water levels following its removal. But that data was not included in estimates used by the agency and the Army Corps of Engineers during a September 2011 public hearing in Riverside.

The Army Corps has since publicly apologized for its failure to disclose that information. As a result, the village demanded an independent review of the river flow data. That independent review was done by the U.S. Geological Survey (USGS), and its findings were presented at a special meeting on June 7.

The USGS numbers more or less confirmed those of the Office of Water Resources, although the figures don’t align exactly.

Hofman Dam