33 CFR §2.36

1. (a)Except as provided in paragraph (b) of this section, navigable waters of the United States, navigable waters, and territorial waters mean, except where Congress has designated them not to be navigable waters of the United States:
   1. (1)Territorial seas of the United States;
   2. (2)Internal waters of the United States that are subject to tidal influence; and
   3. (3)Internal waters of the United States not subject to tidal influence that:
      1. (i)Are or have been used, or are or have been susceptible for use, by themselves or in connection with other waters, as highways for substantial interstate or foreign commerce, notwithstanding natural or man-made obstructions that require portage, or
      2. (ii)A governmental or non-governmental body, having expertise in waterway improvement, determines to be capable of improvement at a reasonable cost (a favorable balance between cost and need) to provide, by themselves or in connection with other waters, as highways for substantial interstate or foreign commerce.
2. (b)Navigable waters of the United States and navigable waters, as used in sections 311 and 312 of the Federal Water Pollution Control Act, as amended, 33 U.S.C. 1321 and 1322, mean:
   1. (1)Navigable waters of the United States as defined in paragraph (a) of this section and all waters within the United States tributary thereto; and
   2. (2)Other waters over which the Federal Government may exercise Constitutional authority.