The US EPA recently released the final draft of its report on the Connectivity of Streams and Wetlands to Downstream Waters. The purpose of the report is to summarize current scientific understanding about the connectivity and mechanisms by which streams and wetlands affect the physical, chemical, and biological integrity of downstream waters. The focus of the review is on surface and shallow subsurface connections of small or temporary streams, nontidal wetlands, and certain open waters. The report stresses that it neither considers nor sets forth legal standards for Clean Water Act (CWA) jurisdiction, nor does it establish EPA policy.

In 2006, the US Supreme Court decided Rapanos v. United States, where it held that a geographically isolated body of water can be regulated under the CWA only if it has a “significant nexus” to “navigable waters” of the United States. The meaning of “significant nexus” was never clarified. The report on connectivity is meant, in part, to provide insight on this question.

According to the report, scientific evidence “unequivocally” demonstrates that streams, individually or cumulatively, exert a strong influence on the integrity of downstream waters. EPA found that wetlands and open waters in riparian areas and floodplains are physically, chemically, and biologically integrated with rivers via functions that improve downstream water quality. These functions include: the temporary storage and deposition of channel-forming sediment and woody debris; recharge of groundwater sustaining river baseflows; storage of floodwater; retention and transformation of nutrients, metals, and pesticides; and export of organisms or reproductive propogating materials to downstream waters. In addition to providing effective buffers to protect downstream waters from point source and nonpoint source pollution, wetlands and open waters form integral components of river food webs, providing nursery habitat for breeding fish and amphibians, colonization opportunities for stream invertebrates, and maturation habitat for stream insects.

The report recognizes that watersheds are integrated at multiple spatial and temporal scales by flows of surface water and groundwater, transport, transformation of physical and chemical materials, and movements of organisms. Connectivity of streams and wetlands to downstream waters occurs along a continuum that can be described in terms of frequency, duration, magnitude, timing, and rate of change of biotic fluxes to downstream waters. Variations in the degree of connectivity influence the range of functions that streams and wetlands provide. Thus, the incremental effects of individual streams and wetlands are cumulative across entire watersheds and must be evaluated in the context of other streams and wetlands.