CHICAGO — Illinois Governor Pat Quinn declared a state of emergency as state agencies prepared to provide assistance to local governments throughout the northern half of Illinois dealing with severe river and flash flooding. According to the Metropolitan Water Reclamation District of Greater Chicago (MWRD), approximately five inches of rain had fallen across the entire region and the Tunnel and Reservoir system, which currently is comprised of 109 miles of tunnel and two reservoirs, is 100 percent full. The entire tunnel system holds 2.3 billion gallons.

The Mainstream Tunnel was 100 percent full at 12:31 a.m. Thursday, April 18, and the Des Plaines Tunnel was 100 percent full three hours later at 3:30 a.m.

When the Chicago area waterway levels are higher than Lake Michigan and predetermined elevations are reached, the MWRD opens control structures to move as much water as possible out of the system. This provides overbank flooding protection as well as more capacity for stormwater.

According to MWRD’s website, “the district adopted the Tunnel and Reservoir Plan (TARP) in 1972 as the Chicago area’s plan to cost-effectively comply with federal and state water quality standards in the 375 square miles combined sewer area consisting of Chicago and 51 suburbs. TARP’s main goals are to protect Lake Michigan — the region’s drinking water supply — from raw sewage pollution; improve water quality of area rivers and streams; and provide an outlet for floodwaters to reduce street and basement sewage backup flooding.

“Phase I of TARP, intended primarily for pollution control, is made up of four distinct tunnel systems: Mainstream, Des Plaines, Calumet, and Upper Des Plaines. After a storm event, pumping stations dewater the tunnel systems as Water Reclamation Plant (WRP) capacity becomes available, making the tunnel and reservoir capacity available for the next storm event. All captured combined sewer flow pumped to the WRP receives full secondary treatment prior to being discharged to the waterway pursuant to the National Pollutant Discharge Elimination System permits.

“Construction of the Phase I tunnel systems commenced in 1975. The tunnel systems were put into service as portions were completed, starting in 1985. By 2006, all of Phase I was completed and in operation. The total system consists of 109.4 miles of deep, large-diameter, rock tunnels providing 2.3 billion gallons of volume to capture combined sewer overflows (CSOs) that previously discharged at hundreds of outfall locations.

“Phase II of TARP consists of reservoirs intended primarily for flood control, but it will also considerably enhance pollution control benefits being provided under Phase I. … When all three reservoirs are completed, the reservoirs will increase the TARP system storage volume to 17.5 billion gallons. The 350 million-gallon Majewski Reservoir was completed in 1998 at a cost of $45 million. The McCook Reservoir is currently under construction and, when completed, will have a total capacity of 10 billion gallons. Phase 1 of the reservoir is planned to be completed by 2017.

The Thornton Reservoir is being constructed in two stages. The first stage, a temporary 3.1 billion-gallon Natural Resources Conservation Service (NRCS) reservoir called the Thornton Transitional Reservoir, was completed in March 2003 in the West Lobe of the Thornton Quarry. This reservoir provides overbank flood relief for nine communities and has captured over 26 billion gallons of flood water. The second stage is a permanent 7.9 billion-gallon combined NRCS/ Chicagoland Underflow Plan (CUP) reservoir called the Thornton Composite Reservoir, to be located in the North Lobe of the Thornton Quarry. The Thornton Composite Reservoir is planned to be completed by 2015 and will provide $40 million per year in benefits to 556,000 people in 15 communities.”