What are the effects of controlling water at this scale?
In addition to the localized effects of flooding and overflows, there are also continental-scale consequences of controlling water in this way. Although physically in the Great Lakes watershed, the City has engineered itself to take advantage of both watersheds. Chicago's hydraulic engineering is a one way pipeline of water out of the Great Lakes watershed to the Gulf of Mexico.
Chicago withdraws its water use from Lake Michigan, and once used, combines this with rainwater falling on the surface -- all of that water is combined and exported outside the City to the west. Such a rapid and vast rainwater collection is drying out the City, instead of allowing water to seeking the ground at a local level in its own watershed, helping to replenishing the Lake. The other challenge is pollution -- once rainwater and wastewater are in the system, any overflows of untreated wastewater leaving Chicago travel through downstream waterways in Illinois and Central Midwest states, through the DesPlaines River, Illinois River, Mississippi River, and to the Gulf of Mexico.
Depending on water quantities and climate conditions, the travel times of exported water vary from a couple week to several weeks. When water exports are the result of combined sewer overflows, the water contains waste materials such as e coli and phosphorus levels, as well as commercial and industrial toxins, which impact water quality downstream. 5% of the hypoxia zone (large region of water containing little to no oxygen) in the Gulf has been attributed to Chicago alone.