What waters are subject to federal regulatory protection by the US Clean Water Act (CWA)? Over the 43 y since passage of the CWA (formally the 1972 Federal Water Pollution Control Act [33 U.S.C. §§1251–1387]) little clarity and less consensus has emerged regarding this fundamental question. However, on 29 June 2015, the US Environmental Protection Agency (EPA) and the US Army Corps of Engineers (ACE) published the Clean Water Rule (https://federalregister.gov/a/2015-13435) to clarify which waters are subject to CWA protection. Publication of the Clean Water Rule created immediate, passionate political debates, largely focused on whether EPA and the ACE overstepped their jurisdiction. Within 5 d of publication of the rule, attorneys general from 31 states planned to ask EPA to delay implementation of the Clean Water Rule for at least 9 mo (http://www.courthousenews.com/2015 /08/03/thirty-one-states-fight-clean-water-rule.htm). As of 10 August 2015, 13 states had asked a federal judge to block the rule (http://www.mercurynews.com/breaking -news/ci_28615224/13-states-want-judge-block-new-federal -water). A Google® search on 13 August 2015 based on the keywords “Clean Water Rule” and “lawsuits” produced 22,500 results. As of 8 September, when this paper went to press, the rule was in effect in 37 states, and was stayed by preliminary injunction in 13 states (Alaska, Arizona, Arkansas, Colorado, Idaho, Missouri, Montana, Nebraska, Nevada, New Mexico, North Dakota, South Dakota, and Wyoming) (http://www2.epa.gov/cleanwaterrule/clean-water -rule-litigation-statement). In bright contrast to the well publicized, negative political reaction, the aquatic resource science and management communities appear to largely support the rule (e.g., http://www.freshwater-science.org/Policy /files/SFS_WOTUS_EPA-HQ-OW-2011-0880.pdf).