News Updates

Strengthening regional resilience amid escalating flood challenges

Flooding is one of the most significant and persistent climate challenges facing northeastern Illinois. Over the past decade, the region has experienced numerous flood events that damaged homes and businesses, disrupted daily life, and put people at risk. These impacts are already widespread, and climate change is expected to make flooding risks worse in the coming decades.

In a continued effort to help tackle current and future flooding challenges, CMAP updated the region’s urban and riverine flood susceptibility indexes (FSIs).

The FSIs measure the region’s susceptibility to flooding using a 1-10 scale, with 1 being the least susceptible and 10 being the most susceptible. Together, they provide an understanding of flood susceptibility across the region and highlight priority areas for focused stormwater management planning and flood resilience investments. The urban FSI illustrates flood susceptibility caused when rain overwhelms an area’s stormwater infrastructure system, while the riverine FSI illustrates flood susceptibility caused by water overflowing the banks of nearby waterways.

Key findings

Cook County has some of the highest susceptibility to flooding, compared to the rest of the region. Within the county, Chicago has some of the highest urban flood susceptibility scores, along with inner-ring suburbs to the south, west, and northwest.

Communities with the highest riverine flood susceptibility are located along some of the region’s major rivers and their tributaries. This includes the Des Plaines River and tributaries (in west Cook, east DuPage, and Will counties), along the Little Calumet River and tributaries of the Cal-Sag Channel in south Cook County, as well as the upper reaches of the North Branch Chicago River in northeast Cook and southeast Lake counties.

Older, denser communities are more susceptible to both urban and riverine flooding. These areas are largely built out, with lots of impervious cover from rooftops and pavement. They often rely on combined sewer systems, and/or were developed before stormwater management requirements were put in place. Most of Chicago and Cook County fit this description, as well as other larger, older cities in the region, such as Aurora, Elgin, and Joliet.

How are the indexes created?

The indexes are created by looking at places where flooding has occurred and analyzing underlying conditions that could result in flooding. The underlying conditions or “flood-related factors” include whether an area is served by a combined sewer system, was developed before stormwater management requirements, or has lots of pavement and rooftops that limit rainwater from soaking into the ground. The analysis then assigns similar scores to places with similar conditions.

Many places across the region experience recurring flooding, but it’s important to underscore that having a higher score doesn’t necessarily mean that it will flood. It just means that these areas could experience flooding in the future, based on past flooding and underlying conditions.

What’s new in the 2026 indexes?

While the approach used to create the indexes remains the same, CMAP made some key updates:

Newer flood data

Over 300,000 unique reported flood locations were used to create the indexes (nearly doubling the number of flood locations used for the indexes in 2017). New flood data accounts for the most recent major floods that impacted Cook County communities in July and September of 2023 and July 2024 (which also affected Will County).

Expanded geographic coverage

The urban index was expanded to include all developed areas, both outside and inside floodplain areas, acknowledging that communities within floodplains can also experience urban flooding.

The riverine index was expanded to cover developed areas within FEMA’s 500-year floodplain and the Metropolitan Water Reclamation District’s 500-year inundation area, both of which have a 1 in 500 chance of flooding every year. Because FEMA’s floodplain mapping is outdated and does not take climate change into account, many experts recommend using the 500-year boundary to communicate the riverine flood risk more commonly experienced today.

How can the region use the indexes?

While the FSIs are not precise flood-mapping tools, they do provide a more accurate and up-to-date understanding of flooding challenges across northeastern Illinois and can be used to advance stormwater management and flood resilience investments.

CMAP encourages stormwater agencies, community planners, and other stakeholders to use the indexes to:

• Screen for potential flooding challenges when developing comprehensive, corridor, and other local plans.
• Identify areas of further study or where improved stormwater management might be most beneficial in a community.
• Analyze who is most vulnerable to the impacts of flooding by overlaying data on age, disability, income, as well as other health and environmental factors.
• Prioritize flood resilience investments, including nature-based solutions (or green infrastructure).
• Engage communities to help benchmark the data and gather local knowledge to support investment decisions.

Download the 2026 urban and riverine FSIs from CMAP’s Data Hub or view the FSIs through an interactive web map. You can learn more about the updates and methodology in the 2026 FSI report.