The choice of whether to send floodwaters contaminated with raw sewage into our drinking water supply or allowing them to flow through our downtown streets is the central dilemma in “A Battle Between a Great City and a Great Lake,” a New York Times article by Dan Egan.
The story reads like a dystopian climate drama, but Chicagoans take this choice for granted because the Lake Michigan locks have been opened once a year on average over the last 37 years of record (Figure 1, below) to relieve the swollen river during extreme floods and associated combined sewer overflow events. But should we take this choice for granted?
The Chicago River originally flowed into Lake Michigan and was reversed by an engineering feat of the century with the opening of the Chicago Sanitary and Ship Canal in 1900. Therefore, modern-day river reversals are a reversal of the engineered reversal back to the natural flow path. Modern river reversals occur when the river water level is higher than the lake water level, which can happen when the lake level is at record low levels or when the river floods to record high levels.
The setting of Egan’s battle story is May 17, 2020, at the Chicago locks on the fourth day of heavy rain.
The reservoirs and tunnels built over four decades to manage flood waters entering our old combined sanitary and stormwater sewers were full and raw sewage was overflowing directly to the Chicago River and threatening to flood the city. Under such conditions, the locks are usually opened to release raw sewage-contaminated stormwater into Lake Michigan, which is our drinking water supply.
However, the Lake Michigan water level was at a record high and well above the river, so opening the locks would only increase flooding downtown. Chicago Harbor Lockmaster Tyrone Valley and his crew had to wait until the Chicago River water rose above the lake level. Then they rescued our city by opening the locks to send contaminated floodwaters out to Lake Michigan and closing them just in time to prevent lake water from sloshing back into the river.
Just seven years earlier, the water level in Lake Michigan was a record six feet lower, and experts estimated that the river could have reversed if the lake level dipped a mere six inches below the 2013 level. Egan claims that if a two-foot storm surge occurs (compare to the maximum recorded 10-foot seiche) at high lake level, waves would overtop the lock gates.
Lake level fluctuations are predicted to continue in the future with shorter times between high and low water levels. Potential causes cited by Egan include competing forces of increased rainfall, increased evaporation when the lake is not ice-covered and decreased evaporation during polar vortex winters when the lake ice area increases. All these factors are linked to rising average air temperature (+1.2 Fahrenheit from 1991 to 2021).
The fundamental premise of emergency preparedness planning is to accept that failure will happen and plan to shorten recovery time. Our combined sewers convey both industrial and domestic water to seven treatment plants located throughout Cook County (Figure 2, below).
If industrial wastewater were segregated and recycled onsite, industrial chemicals would be removed from our sewers and potential overflow to our drinking water supply during extreme weather events. Onsite industrial reuse would also decrease base flow in our combined sewers, freeing some capacity for extreme stormwater events.
While there is no single solution to climate change, water reuse is a step toward adapting to increases in extreme weather. Water reuse is the future, even in non-drought areas, for the protection of our drinking water source.
Sharon Waller is a licensed Environmental Engineer and Principal at Sustainable Systems LLC – Consulting in Chicago.