Environmental Data: Flood Susceptibility Indexes

Prioritizing areas for flood mitigation

The Chicago Metropolitan Agency for Planning (CMAP) developed the urban and riverine flood susceptibility indexes (FSIs) to identify priority areas for flood mitigation.

The FSIs identify developed areas within northeastern Illinois that are most likely to experience flooding impacts. Susceptibility is scored on a scale of 1 to 10, with 1 being the lowest susceptibility and 10 being the highest.

Since there are different factors that contribute to flooding, there are two indexes — and together they provide an understanding of flood susceptibility across the region:

• Urban FSI: illustrates flood susceptibility caused when precipitation and stormwater exceed the capacity of an area’s stormwater infrastructure system
• Riverine FSI: illustrates flood susceptibility caused by water overflowing the banks of nearby waterways

Download the 2026 urban and riverine FSIs from CMAP’s Data Hub or view them through an interactive web map. You can also learn about FSI methodology, including the most recent updates and flood factors used for each index, in the 2026 FSI report. Raw FSI scores are available upon request.

Urban flood susceptibility index

The 2026 urban FSI includes all developed areas of the region, both within and outside of floodplains.

Regional urban FSI Opens in a new tab

View rescaled maps that show the urban FSI relative to each county rather than the whole region.

Cook County Opens in a new tab DuPage County Opens in a new tab Kane County Opens in a new tab Kendall County Opens in a new tab Lake County Opens in a new tab McHenry County Opens in a new tab Will County Opens in a new tab

Riverine flood susceptibility index

The 2026 riverine FSI covers developed areas in the region within the Federal Emergency Management Agency’s (FEMA) 500-year floodplain or the Metropolitan Water Reclamation District’s 500-year inundation layer within Cook County.

Regional riverine FSI Opens in a new tab

View rescaled maps that show the riverine FSI relative to each county rather than the whole region.

Cook County Opens in a new tab DuPage County Opens in a new tab Kane County Opens in a new tab Kendall County Opens in a new tab Lake County Opens in a new tab McHenry County Opens in a new tab Will County Opens in a new tab

Use the flood susceptibility indexes

While the FSIs are not precise flood models or floodplain mapping efforts, they do provide a broad overview of the region’s flooding challenges and serve as tools to advance stormwater management and flood resilience activities.

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 other 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.

View the Guide to Flood Susceptibility and Stormwater Planning to learn other ways to use the indexes in your planning activities.

Methodology

The FSIs analyze places where flooding has occurred and review the conditions that could result in flooding. The underlying conditions — or “flood-related factors” — include features such as 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.

With data assistance from FEMA, counties, the City of Chicago, and other regional partners, CMAP used over 300,000 reported locations to identify where flooding occurred. 

While many places across the region experience recurring flooding, 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.