Linking wetlands to relatively permanent flowing waters: a conterminous United States geospatial analysis

Clean Water Act (CWA) coverage extends to certain wetlands, including those with a continuous surface connection to relatively permanent tributaries. However, limited information is available to estimate the national extent of wetlands potentially afforded CWA coverage. To address this data gap, we identified conterminous US (CONUS) palustrine wetlands connected by an exploratory 150 m buffer to a high-resolution CONUS-wide stream network hydrography dataset, a reasonable and defensible proxy for a continuous surface connection between wetlands and relatively permanent waters. Nationally, 79% (23.1 Mha) or 66% (19.3 Mha) of the nation's CONUS freshwater palustrine wetlands are potentially connected to the stream network, depending on whether a more inclusive or exclusive flow permanence network is analyzed. Conversely, 21% (6.1 Mha, roughly the area of West Virginia) or 34% (9.9 Mha, greater area than Indiana) of CONUS wetlands may be outside this buffer. Results for individual states varied widely based on stream and wetland density. States with a low relatively permanent stream density had fewer buffer-connected wetland resources (e.g., 21% in North Dakota). Similarly, wetlands in southwestern states and other states with abundant ephemerally flowing streams were also not connected via the applied buffer. Geospatial data limitations and assumptions (e.g., omission errors, presumed presence of a surface connection) suggest the estimated extent of wetlands with potential continuous surface connections to federally covered waters are likely to be substantially smaller than reported here. Nonetheless, the analyses herein provide insights for local, state, and tribal stakeholders to consider in managing their wetland resources.