There is , perhaps , no other field of engineering that develops in the course of construction so many new conditions and unexpected obstacles , and when the en- gineer finds it necessary to make radical changes in the plans , even though involving heavy expenditures and sub- jecting himself to severe criticism , it is his bounden duty to make these changes . The writer had the honor of being Chief Engineer of the Aqueduct Commission of the City of New York , from January Ist , 1900 , to October 14th , 1903. During his incumbency the new Croton Dam was under construc- tion . The purpose of this paper is to explain , as briefly as possible , the original plan of this structure and the im- perative necessity of its modification , and to answer a general demand made upon the writer that would seem to indicate a very widespread interest in these rotable changes . Had the work continued under the original plan it might have resulted not only in the failure of the structure itself but have been the cause of immeasurable loss to this great city of its supply of water in addition to an appalling destruction of life and property . The boroughs of Manhattan and Bronx , of an estimated population of 2,200,000 are dependent almost entirely upon the Croton River for their supply of water ; in fact , they have no other supply , except the comparatively small quantity ( 20,000,000 gallons per day ) from the Bronx and Byram Rivers , small as compared with 290,000,000 gallons , the present daily consumption of these two boroughs . The natural flow of Croton River in dry seasons was judged insufficient to supply the city as early as 1843 , now more than 60 years ago , so Croton Lake of a capac- ity of two billion gallons , was then created by building the old Croton Dam .