It is well up on the slope of the valley , its base being about 100 feet higher than the bed of the river . The substructure of this gatehouse is described as follows : It has four water chambers , 74 feet deep and about 12 feet square . Two on the southerly side of the structure are to control the flow of the water in the old Croton aqueduct which passes through this gatehouse . The other two on the northerly side , are to control the flow of water direct from the res- ervoir to the old aqueduct below the dam . These two chambers are connected with the reservoir by three brick inlets , 10 feet in diameter . And thy are to hold water to a depth of 54 feet , yet the outer walls ( which are en- closed in an artificial embankment ) have a uniform thick- ness , from the base to top of but eight feet , while safe construction demands that they should be about four times that thickness at the base . These walls which had been built to a height of fifty feet , would not have sufficient strength to resist the water pressure that would come against them , and their de- struction might result in a great losss of water from the reservoir and the demolition of the embankment , by the rush of water in falling a height of 150 feet to reach the valley below . On Dec. 18 , 1902 , on the writer's recom- mendation the Aqueduct Commissioners approved a plan providing for the reinforcement of these walls . The preliminary estimate of the cost to construct the dam under the original plan was $ 4,150,573 . This will be increased to about $ 6,400,000 .