All of the rain that has fallen across the Chicago area in recent days has officials keeping an eye out for a problem.

The deep tunnel system is filling up once again, especially the stormwater reservoir in McCook.

That's why, in part, Monday marked another Overflow Action Day, a day when the Metropolitan Water Reclamation District asks people to use less water to ease strain on the system, known as the TARP.

"Delay washing dishes, delay running the dishwasher. They could take shorter showers, anything they can do. To keep water out of the system, because in situations like this, every drop counts," John Murray, MWRD executive director, said.

The system's second largest reservoir, McCook, is 95% full after heavy storms swept through the area Sunday night.

The McCook Reservoir reached capacity five times between 2021 and 2025. This year alone, it's filled up seven times, as recently as last month.

The Thornton Reservoir in the south suburbs is around 60% full.

A third reservoir, Majewski in Elk Grove Village, is much smaller.

NBC 5 spoke with Max Berkelhammer, an environmental science professor at University Illinois Chicago, about the challenges facing the TARP system.

"When those are at capacity, there is no more capacity, which means the tunnel is going to fill up and that water is going to come back into the streets," Berkelhammer said.

Riverside is one of the communities that relies on the McCook Reservoir.

Village trustee Alex Gallegos said flooding has been a concern for decades. "We're right by the river so of course there's always an issue with high rainstorms," he said.

NBC 5 has reached out to the Army Corps of Engineers for an update on the project.

In addition to people using less water when heavy storms hit, Berkelhammer said more green infrastructure could help ease the burden on the system.

"It's very hard to continually upgrade and increase the capacity of the system to keep up with the changing climate," Berkelhammer said.