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Metropolitan Water Reclamation District of Greater Chicago

Controlling the Flow

We control the rate of water flowing out of the CAWS at the downstream end and can control the rate of lake water entering through sluice gates (underwater doors) at the upstream control structures. The level of the CAWS is normally several feet below Lake Michigan so water flows in from the lake by gravity. The system is sloped to flow downhill to Lockport so water flows away from the lake.  

Normal condition

A diagram of the Chicago Area Waterway System in normal conditions

In normal conditions, water flows in from the lake, WRPs and local streams. Water flows out at Lockport Powerhouse, generating hydroelectricity. 

When a storm is forecast

Before a storm, we lower the level of the CAWS to make room for more water. We do this by increasing the flow out at Lockport and closing the sluice gates at the lake. We can’t lower the water too much, though. We are required to keep the water deep enough for boat traffic. 

Moderate storm

A diagram of the Chicago Area Waterway System in moderate rainfall

In a moderate storm, we increase the flow out at Lockport by opening additional sluice gates. Flow from local streams and WRPs increases in rainy weather. In some cases, sewers may overflow to the system. The sluice gates at the lake are closed - we don’t want any extra water entering the system!

Extreme storm

A diagram of the Chicago Area Waterway System in severe rainfall

In an extreme storm, water can enter the CAWS faster than it can flow out to Lockport. If the water at the control structures is higher than the lake and the system is in danger of overtopping its banks, we can open sluice gates and locks to release flood water to the lake. This is commonly called a “reversal” but in these cases the river is actually flowing both directions!

Limits on the effect of waterway operations

Although CAWS operations can help prevent overbank flooding from the system, there are limits to what it can do.

  1. Opening the gates and locks at the lake can only help reduce flooding if the CAWS is well above the level of the lake. If control structures are opened when the lake is higher than the river, more water will enter the river.
  2. CAWS operations do not affect area waters that are not part of the system. For example, the Des Plaines River and the North Branch of the Chicago River upstream of the North Shore Channel are not part of the CAWS.
  3. Like all rivers, the CAWS has a maximum flow rate. If more water enters it than can flow through it during heavy rains, the water level will rise even if all control structures are letting out water as fast as possible. 
  4. Street flooding and basement backups often happen when more water enters small neighborhood sewers than can flow through them. The CAWS is usually far downstream of these neighborhood sewers and is not connected to them.