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Mile zero is in the District of Columbia near where Rock Creek flows into the Potomac River. This spot is the "tidewater lock", where canal boats entered the river - the northernmost navigable part of the Potomac. Another name was the "watergate." Just across Virginia Avenue from this spot is the apartment/office complex by the same name, unfortunately a name that now has a different historical meaning.
Best wishes for Steve's adventure!
Dixie Sommers
Alexandria, VA