The rain over the July Fourth holiday weekend overwhelmed a massive system meant to handle Chicago's stormwater, causing untreated water to be dumped into area rivers.

Multiple rounds of heavy rain fell between July 2 and July 4, leading to flash flooding in streets and rising river levels across the greater Chicago region, with some municipalities reporting 8 to 10 inches of rain over the 48-hour period.
According to the MWRD, its two largest Deep Tunnel reservoirs are nearly at capacity.

Thornton Reservoir nearly full after heavy storms over weekend

The Thornton Reservoir, which serves many surrounding south suburbs, is nearing capacity after storms swept through the area this past weekend. FOX Chicago’s Joanie Lum has the latest on its condition.

This weekend’s heavy rainfall in the Chicago area has helped to fuel record amounts of stormwater being stored in Cook County’s reservoirs, nearly 20,000 Olympic-size swimming pools’ worth.

A soggy start to summer with near-record rainfall has left Chicago’s Deep Tunnel stormwater retention system almost full.
The Deep Tunnel, which was constructed over the course of more than five decades, is a system of underground tunnels and reservoirs that captures and stores stormwater and sewage that could otherwise overflow into waterways.