'Waters of the United States' (WOTUS): Current Status of the 2015 Clean Water Rule [December 12, 2018] [open pdf - 1MB]
"The Clean Water Act (CWA) is the principal federal law governing pollution of the nation's surface waters. The statute protects 'navigable waters,' which it defines as 'the waters of the United States, including the territorial seas.' The scope of the term waters of the United States, or WOTUS, is not defined in the CWA. Thus, the Army Corps of Engineers and U.S. Environmental Protection Agency (EPA) have defined the term in regulations several times as part of their implementation of the act."
CRS Report for Congress, R45424
Congressional Research Service: https://crsreports.congress.gov/