Subject to paragraph (2) of this subsection, the consent of Congress is granted under section 401 of title 33 to the Washington Suburban Sanitary Commission to construct a water diversion structure, with an elevation not to exceed one hundred and fifty-nine feet above sea level, from the north shore of the Potomac River at the Washington Suburban Sanitary Commission water filtration plant to the north shore of Watkins Island.

The structure authorized by paragraph (1) of this subsection, may not be constructed until the Secretary of the Army, acting through the Chief of Engineers, and the State of Maryland, the Commonwealth of Virginia, the Washington Suburban Sanitary Commission, and such other governmental authorities as the Secretary of the Army, the State of Maryland, and the Commonwealth of Virginia deem desirable signatories enter into a written agreement providing an enforceable schedule for allocation among the parties to such agreement for the withdrawal of the waters of that portion of the Potomac River located between Little Falls Dam and the farthest upstream limit of the pool of water behind the Chesapeake and Ohio Canal Company rubble dam at Seneca, Maryland, during periods of low flow of such portion of such river.