Abstract
Wetlands provide a wealth of ecosystem services, including flood mitigation, wildlife habitat, and a natural ex-post solution to surface water pollution from agricultural runoff (Taylor and Druckenmiller 2022; Karwowski and Skidmore 2025). Thanks to their natural benefits, wetland restoration in the United States is supported by programs including the Agricultural Conservation Easement Program. Additionally, wetlands that are part of the Waters of the United States (WOTUS) are protected under the Clean Water Act. Which wetlands are included in WOTUS has been a legal and policy debate for decades and is currently limited to wetlands directly abutting another surface water feature in WOTUS (Supreme Court of the United States 2023). Yet, there is limited causal evidence on the ecological impact of wetlands based on their surface water connection (Cohen et al. 2016).
We test the impact of restored wetlands on downstream water quality based on the distance from the wetland to the nearest protected surface water feature. We link a rich dataset on all wetlands restored under USDA easement contracts in the Mississippi River Basin, streams and rivers mapped in the National Hydrology Database, and harmonized measures of surface water quality from nearly three decades. We use an upstream-downstream model to evaluate the changes in water quality before and after wetland restoration. We find that non-abutting wetlands within one kilometer of a surface water feature significantly reduce phosphorus and nitrogen concentrations downstream. In contrast, abutting wetlands provide only modest downstream water quality benefits, though water quality is only one of the many ecosystem services that wetlands provide. The effects are most pronounced in the Upper Mississippi region, which is a large contributor to the Gulf Hypoxic Zone (Evenson et al. 2021), and the recipient of significant federal, state, and local resources to reduce nutrient loss (Illinois Environmental Protection Agency et al. 2023; Iowa Department of Agriculture and Land Stewardship et al. 2025). Our results suggest that wetlands are an effective tool to improve water quality, and considering their heterogeneous benefits could be pivotal in creating optimal policy.