Why is Chicago area river water reversed into Lake Michigan?

The video is from a previous report.

CHICAGO (CBS) -- On rare occasions of heavy rainfall, the Metropolitan Water Reclamation District officials will reverse the flow of river water into Lake Michigan. 

It has occurred 50 times since 1985, most recently on July 2, when some areas of Chicago got eight inches of rain. 

The most significant "dry spell" was five years in the mid-1990s. 

Why it's necessary

It only happens when Chicago area waterway levels become higher than Lake Michigan. 

Opening the flood gates and locks into the lake reduces river flooding and creates more capacity for stormwater in the area reservoirs. 

Officials keep river water out of the lake to reduce water pollution. 

What is TARP? 

Chicagoland has a system of tunnels and reservoirs--part of the Tunnel and Reservoir Plan (TARP)--to collect stormwater. Wednesday's storms filled the McCook reservoir to 90% of capacity. 

Here is a look at the status of the entire system:

Kirie TARP Status

The Kirie TARP system is comprised of the Kirie Tunnel and Majewski Reservoir.

Kirie Tunnel is 4% full = 2.75 million gallons

Majewski Reservoir = 0% full

McCook TARP Status

The McCook TARP system comprises the Mainstream Tunnel, Des Plaines Tunnel, and McCook Reservoir.

Mainstream Tunnel is 67% full = 803.6 million gallons 

Des Plaines Tunnel is 16% full = 64 million gallons

McCook Reservoir is 90% full = 3.23 billion gallons

Calumet TARP Status

The Calumet TARP system comprises the Calumet Tunnel and Thornton Composite Reservoir.

Calumet Tunnel is 75% full = 470 million galloms

Thornton Composite Reservoir is 19% full = 1.5 billion gallons

List of flooding dates

Here is a listing of the times MWRD took action to send water into Lake Michigan.  

2021 None
2020 5/17-5/18  
2020 5/15  
2019 10/3  
2018 None 
2017 10/14-10/15  
2017 4/29-4/30  
2016 7/24  
2015 6/15-6/16 
2014 6/30-7/1  
2013 4/18-4/19  
2012 None 
2011 7/23  
2011 5/29  
2010 7/24  
2009 6/19  
2009 3/8  
2009 2/26-2/27  
2008 12/27-12/28  
2008 9/13-9/16 Record 11 million gallons flowed into the lake. 
2007 8/23-8/24  
2006 None  
2005 None 
2004 None  
2003 None  
2002 8/22  
2001 10/13  
2001 8/31  
2001 8/2 
2000 None 
1999 6/13  
1998 None 
1997 8/16-8/17  
1997 2/20-2/22  
1996 7/17-7/18  
1995 None 
1994 None  
1993 None  
1992 None  
1991 None 
1990 11/27-11/28  
1990 8/17-8/18 
1990 5/9-5/10  
1989 8/3-8/4  
1988 None 
1987 8/25-8/26  
1987 8/13-8/14  
1986 10/3  
1985 8/6  
1985 3/4 

Flooding map

Wednesday's storms have the potential to be severe, with damaging winds, large hail, and possible flooding. 

Right now, here is a look at Chicago homes that have reported basement flooding or water in the street on Wednesday. The West Side has been hit hard.