D.C. Code § 10-145

In order to protect Rock Creek and its tributaries, none of the moneys appropriated on or before June 7, 1924, for the opening, widening, or extending of any street, avenue, or highway in the District of Columbia shall be expended for the opening, widening, or extension of any street, avenue, or highway which shall or may in the judgment of the Mayor of the District of Columbia permanently injure or diminish the existing flow of Rock Creek or any of its tributaries, nor shall permission so to do at private expense be granted to any private person or corporation except by the joint consent and approval of the Mayor of the District of Columbia and the Director of the National Park Service.