A national policy with local consequences: Quantifying the downstream effects of Sackett on Illinois wetlands and communities.

The 2023 Sackett v. EPA US Supreme Court ruling narrowed Clean Water Act (CWA) protection for wetlands by requiring a "continuous surface connection" to "waters of the United States" (WOTUS). As most states have limited or no legal wetland protections beyond the CWA, the ruling has left millions of US wetlands, with $11.2-$67.0 billion in flood-control benefits, vulnerable to impacts. To assess the ruling's implications for Illinois, we integrated geospatial datasets to identify state wetlands that could be federally non-jurisdictional (non-WOTUS) across 96 policy scenarios with different requirements for wetland spatial and temporal connectivity to relatively permanent waters (RPWs). Although agency requirements for wetland flood frequency were the largest source of policy uncertainty, we estimated that up to 286,342 ha (72.1 %) of remaining state wetland area could be non-WOTUS if wetlands must have at least seasonal flooding for CWA protection. Despite predicting that up to 227,642 ha (79.5 %) of non-WOTUS area is unprotected, we estimated that more non-WOTUS area is protected than previous studies by accounting for county-level regulations. By further evaluating the relationship between unprotected wetland area and risk of climate-driven natural disasters, we found that county-level regulations limit the potential for wetland losses to increase 30-year flood risk in northeastern Illinois. Outside this region, unprotected wetland area has a strong positive correlation with property share at risk of flooding in 30 years and with expected losses in population and building value. Thus, enhancing protections for wetlands that are intermittently dry or geographically isolated from WOTUS could increase the resilience of lower elevation and latitude floodplain communities to climate change.