The city has no auxiliary supply , and no lakes or rivers that could be used as such in case of an emergency , hence an accident to a reservoir might result in so great a loss of water as to cause a water famine , truly styled one of the world's greatest disasters . The new Croton dam is located about thirty - seven miles from the Central Park reservoirs , it is on the Cro- ton River , about two and one - half miles below the old Croton dam . Its purpose is to enlarge Croton Lake , in- creasing its capacity from two to thirty - two billion gallons and its length from five and one - half to nineteen and three- quarter miles . The old dam will be sumberged , as the flow line will be raised thirty - six feet . The clearing of this great basin for the reservoir , twen- ty miles in length , was in itself an extraordinary under- taking , embracing the clearing of all timber , the removal of three villages , numberless buildings , farm houses and cemeteries and the abandonment and reconstruction of railroad tracks , telegraph and telephone lines , highways and bridges . The work of constructing the new Croton dam com- menced October 1st , 1892. It calls for a very large quantity of masonry ; in fact , there will be no less than 800,000 cubic yards . In this connection it may be of in- terest to note that it will be the largest block of masonry in the world , excepting the Egyptian pyramids . There are three distinct features of the dam , the spill- way at the north end ; the main stone dam crossing the valley and the embankment at the south end .