Science at the boundaries: scientific support for the Clean Water Rule

The US Environmental Protection Agency (EPA) and US Army Corps of Engineers have promulgated a definitional rule to clarify the scope of waters of the United States protected under the Clean Water Act (CWA). The Clean Water Rule, published in June 2015, defines 8 categories of waters for jurisdiction and expressly excludes certain features from jurisdiction, based on law, science, public input, and 40+ y of experience implementing the CWA. It also defines terms used in regulation including, for the first time, tributary, neighboring, and significant nexus. Much of the scientific basis for this rule is contained in a report titled Connectivity of Streams and Wetlands to Downstream Waters: A Review and Synthesis of the Scientific Evidence developed by EPA's Office of Research and Development to inform the rulemaking process. As a scientific review, the report does not consider or set forth legal standards for CWA jurisdiction. Rather, it summarizes current scientific understanding of the connections and functions by which small or temporary streams, nontidal wetlands, and other source waters, singly or in aggregate, exert a strong influence on the chemical, physical, or biological integrity of waters protected by the CWA. It is the result of a multiyear collaboration by scientists and nonscientists working across disciplinary and organizational boundaries to synthesize the best available science in response to evolving policy needs. Here I summarize the major conclusions from the report, the influence of science on policy decisions in the final rule, and some challenges of synthesizing, interpreting, reviewing, and communicating the large body of scientific evidence needed for development of this rule.