Is Williamson County, IL in a flood zone?
FEMA's National Risk Index puts Williamson County's flood hazard at the 69.9th percentile nationally — below the national norm for flood hazard — driven by inland rivers and flash flooding rather than the coast — FEMA's model puts a flood event here at about 2 times a year. This describes the county, not any one street — FEMA only publishes exact parcel zones through a live map service.
Williamson County's flood-insurance compliance gap
Homes with a federally backed mortgage inside Williamson County's mapped high-risk flood zone (the Special Flood Hazard Area, or SFHA) are legally required to carry an NFIP flood policy. In practice, 14.8% of them do — 106 active policies against 715 structures FEMA counts in that zone, leaving a real compliance gap. Across all residential structures in Williamson County, not just the ones FEMA mandates, 0.5% carry an NFIP policy — most flood damage nationally happens outside the mapped high-risk zone, so that broader number matters too.
Williamson County against the rest of the country
Williamson County sits at rank 988 of 3144 counties on FEMA's flood hazard scale (1 is worst) and rank 815 of 3144 by population (1 is biggest). About 67,064 people live in Williamson County, Illinois, spread over 420.216 square miles.
What to do next if you own or are buying in Williamson County
FEMA counts 29,524 residential structures in Williamson County overall, 141 of them carrying some form of NFIP coverage — but a lender or insurer cares about one parcel, not that county total.
More on how this number was built
The methodology guide shows exactly which FEMA and Census tables feed Williamson County's numbers. The guide to what the Special Flood Hazard Area actually means explains why the mandatory-insurance rule exists and who it applies to.