The Chicago area’s sewer district is busy readying a report on the billions of gallons of stormwater overflow that were released into Lake Michigan as part of its efforts to fight flooding during last weekend’s torrential rains.

The storm, which dumped a record 100 billion gallon rainfall on the Chicago metropolitan area from Friday afternoon to Sunday evening, swelled creeks and caused flooding to homes and businesses in Albany Park, as well as the Northwest and South suburbs.

Terry O’Brien, board president of the Metropolitan Water Reclamation District of Greater Chicago, said at yesterday’s board meeting that the district is working on a report for public release that will provide exact data on the rainfall in last weekend’s storm, as well as the amount of stormwater overflow that was released to Lake Michigan. It will also include data from water quality sampling being conducted by the district.

The district estimates that from last Friday afternoon to 4:15 p.m. Sunday, when its meteorological services issued an “all clear,” approximately 90 billion gallons of water were released into Lake Michigan.

The lake is the main source of drinking water for the region.

“I think what’s remarkable about this to me is the fact that we had this widespread rainstorm over the entire area, amounts of rain we hadn’t seen before in a period of time…and the period of time that we discharged to the Lake,” says District Superintendant Richard Lanyon.

The intensity of the storm “maxed” out the district’s 32 flood control reservoirs and its 109 miles of Deep Tunnel, designed to collect combined sewer overflow from Chicago and 51 suburban municipalities, says district spokeswoman Jill Horist.

As a last resort, the district opened the three locks and gates that discharge storm water overflow into Lake Michigan.

The Wilmette lock, on the North Shore Channel, and the Chicago River Controlling Waterworks, near Navy Pier, were opened on Saturday morning. The gates at the O’Brien lock, at 130th Street and Torrence Avenue, was opened on Saturday evening, in order to provide relief to the Southwest suburbs.

The district closed the locks Wednesday evening.

Horist estimates that water levels in the Chicago River and district pumping operations will not return to normal levels for at least several days.

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