33 CFR 321.2: Definitions.

For the purpose of this regulation, the following terms are defined:

(a) The term navigable waters of the United States means those waters of the United States that are subject to the ebb and flow of the tide shoreward to the mean high water mark and/or are presently used, or have been used in the past, or may be susceptible to use to transport interstate or foreign commerce. See 33 CFR part 329 for a more complete definition of this term.

(b) The term dike or dam means, for the purposes of section 9, any impoundment structure that completely spans a navigable water of the United States and that may obstruct interstate waterborne commerce. The term does not include a weir. Weirs are regulated pursuant to section 10 of the Rivers and Harbors Act of 1899. (See 33 CFR part 322.)