What would happen if 45 inches of rain fell on Chicago?

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Dear Tom,
What would be the effect on Lake Michigan if Chicago received 45 inches of rain as coastal Texas did in August, 2017, with Hurricane Harvey?
— Jim and Ed Schumer, Mount Prospect

Dear Jim and Ed,
If 45 inches of rain fell only on the Chicago area, the effect on the water level of Lake Michigan would be negligible. (Note that Lake Michigan and Lake Huron are actually one huge lake, connected by the Straits of Mackinac.) Much of the rainfall water in 45 inches of rain in the Chicago area would not even enter Lake Michigan. Rather, it would drain southwestward and away from the lake via the Des Plaines and Fox Rivers. Although horrendous flooding would occur in the Chicago area should it receive 45 inches of rain over a few days, the effect on the level of Lake Michigan would be minimal.
If a 45-inch rain occurred across all of the Lake Michigan/Huron system in a few days’ time, the level of the lakes would immediately rise 45 inches from rain falling directly onto the lake surfaces. Additional rises would occur in the days following the rain as water from rivers and streams that drain into the lakes across portions of Wisconsin and Michigan flooded into the lakes. Then, in following weeks as water gradually exited from Lake Huron via the St. Clair River, the lake levels would slowly return to normal.

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