Chicago River Mouth

the locks are key
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And the locks are key

The Chicago Harbor Lock just east of the river mouth is used to control the depth and speed of the river. Along with nearby sluice gates that can shut off or turn on the flow of water, the lock was built to ensure that the river doesn't draw more water from the lake than federal law allows. Controlling how much water is in the river also helps keep it flowing away from Lake Michigan. Water levels in the Chicago River are typically around 1-4 feet below the lake. Without this control system, stormwater flooding into the river or unusually low lake levels could leave the river higher than the lake. If that were to happen, gravity would start to pull water back toward Lake Michigan.

The river does at times spill backwards into the lake. It can become so swollen with stormwater that the lock has to be opened to prevent flooding, briefly returning the river to its original flow.

The lock also makes it possible for boats to safely move between the differing depths. An estimated 50,000 vessels move through the lock each year, with ships and barges carrying approximately 200,000 tons of cargo.
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