Flood Risk in Madison County, Illinois: The American Bottom and the 1993 Flood

The geography: a county built partly on a historic floodplain

The American Bottom is the Mississippi River's floodplain in the Metro East region, running roughly 90 miles from Alton south to the Kaskaskia River, averaging about 5 miles wide and covering roughly 450 square miles (288,000 acres) across Madison, St. Clair, Monroe, and Randolph Counties. Madison County sits at the American Bottom's northern end, which means a meaningful share of the county's lower-elevation land -- particularly around Alton and the river-adjacent industrial cities that grew up along the Mississippi in the 19th and early 20th centuries -- is built on ground the river has flooded before and can flood again.

The 1903 flood and the levee district it created

Madison County's documented flood history goes back at least to 1903, when the Mississippi River flooded the American Bottom and the Madison County levee -- which ran from near the Merchants Bridge at Venice eastward to Mitchell -- broke on June 6. At the time, East St. Louis and the other industrial cities on the American Bottom relied on a patchwork of railroad embankments, not a coordinated levee system, to hold back the river. That flood was consequential enough that the Illinois Legislature passed an enabling act in 1907 to permit formation of the East Side Levee and Sanitary District, covering land in both Madison and St. Clair Counties, with permanent organization following in February 1909 -- one of the direct institutional responses to the 1903 disaster.

1993: when the levee failed from underneath, not over the top

The more consequential flood for anyone evaluating property in Madison County today is the historic Midwest flood of 1993. Heavy, extensive rains across the Upper Midwest pushed the Mississippi and Missouri Rivers to record levels that summer, and in Alton specifically, the river crested 21.7 feet above flood stage. Alton residents and volunteers worked around the clock filling sandbags and reinforcing levees and floodwalls to protect downtown -- but despite those efforts, the river ultimately won, and not in the way people typically picture a flood defense failing.

Rather than the Mississippi simply rising high enough to top the levee, floodwater found its way through the ground and through sewage pipes running underneath the levee, eventually bursting up through the pavement behind it. That failure mode matters for how you think about flood risk here: a levee or floodwall that's tall enough on paper to hold back a given river stage doesn't necessarily protect against water moving underneath it through porous soil or old infrastructure, particularly during a sustained, multi-week high-water event rather than a brief spike. FEMA didn't officially declare an end to the 1993 flood incident period in Illinois until October 22 -- 193 days after it began -- and residents up and down the Calhoun, Jersey, and Madison County riverfronts lost homes in the process.

What Madison County's own floodplain rules require today

Partly as a result of this history, Madison County's floodplain administration -- run through the county's Planning & Development department -- is an active, enforced program rather than a formality. The county maintains at least two Certified Floodplain Managers (CFMs) on staff, and any building permit application for a new structure proposed within a mapped Special Flood Hazard Area must include an Elevation Certificate showing the lowest finished floor at least 2 feet above the Base Flood Elevation, or an approved FEMA Letter of Map Change. Land development within a mapped floodway is prohibited outright unless a permit is issued by IDNR's Office of Water Resources and FEMA provides approval. Illinois currently has 82 counties and 770 communities enrolled in the National Flood Insurance Program (NFIP) with an adopted local floodplain ordinance -- Madison County is one of them, and if the county failed to enforce these standards, its NFIP enrollment itself could be terminated, which would leave every floodplain property owner in the county without access to federally backed flood insurance.

For repeat flood losses specifically, FEMA's Flood Mitigation Assistance Program includes a Severe Repetitive Loss track for properties that meet NFIP criteria as Repetitive Loss or Severe Repetitive Loss -- worth asking about directly if you're looking at an older structure in a flood-prone part of the county with a documented claims history.

What to check before buying

Start with a current FEMA Flood Insurance Rate Map (FIRM) lookup for the specific parcel, which will show whether it sits in a mapped Special Flood Hazard Area and what that means for federally backed mortgage flood-insurance requirements. From there, ask Madison County Planning & Development's Certified Floodplain Managers directly about the parcel's Base Flood Elevation, whether it sits in a mapped floodway (where development is effectively prohibited without an IDNR/FEMA permit), and whether the parcel or a prior structure on it has any Repetitive Loss claims history.

• Get a current FEMA FIRM determination for the specific parcel, not just the general area or municipality
• Ask Madison County Planning & Development whether the parcel is in a mapped floodway (development-restricted) versus a broader Special Flood Hazard Area (development-permitted, with elevation requirements)
• For an older structure near Alton or another American Bottom community, ask directly about a Repetitive Loss or Severe Repetitive Loss designation before assuming standard flood-insurance pricing will apply
• Remember the 1993 lesson: a levee's rated protection height doesn't guarantee protection against water moving underneath it during a sustained high-water event -- ask about local levee-district maintenance and any known seepage or underseepage history near the parcel