Croton Dam

The southern shore of the Croton River at the point where the dam is located, is a gneiss rock; moderately sloping up the hill from the water's edge; but not appearing to extend far into the river. The dam required to be raised forty feet above low water in the river; and it was an important object to arrange the plan, so as to obtain a rock foundation for the masonry.

To effect this, the northern abutment was located as far in the river as the rock was supposed to extend, and from this to build up the dam to the level where the top line intersected the natural slope of the hill, and then to cut the hill away; making an opening sufficient for the waste weir of the dam.

In prosecuting the work, it was found that the rock descended more rapidly into the river than was supposed, which induced the moving of the abutment further towards the hill than at first located; and finally, an artificial foundation had to be made for a small portion of it. It was intended to make the waste of the dam 100 feet, with abutments of eight feet high; but in consequence of the disappointment in regard to the extent of the rock in the river, it was found difficult to obtain the desired length of water-way, and it was concluded to raise the abutment to twelve feet at the lower end, and fifteen feet at the upper end, and allow the water-way to remain an average length of ninety feet for this height. The natural rock formed the southern abutment, and the aqueduct being on this side, the water was conducted to the gateway, at its head, by a tunnel, cut 180 feet through the rock; this allowed the gateway to be located on solid rock, in a situation not exposed to the floods of the river. The water enters the gate-chamber by an archway through the second bulk-head. The gate-chamber is provided with a double set of gates; one set of guard-gates, of cast iron, set in cast-iron frames, and one set of regulating gates, made of gun metal, set in frames of the same material; the gates are all eighteen by forty inches, and there are nine gates in each set. They are all operated by means of wrought-iron screw-rods.

The gate-chamber and bulkheads are constructed of well-dressed masonry, laid up in hydraulic cement.

In the north abutment a waste culvert has been constructed, with suitable gates of cast iron, to draw the water down in the reservoir, at such times as it may be necessary, to facilitate the making of any repairs that may be required and to discharge the river at ordinary times during the construction of the work. From this abutment the old channel of the river was filled by an embankment, with a heavy protection-wall on the lower side, which was raised fifteen feet above the waste weir of the dam, and designed to be fifty feet wide on the top, but was not completed the full width when the unprecedented flood of January, 1841, carried it away. The embankment stood well, and gave no indications of failure, until the water rose to near the surface, and passed through between the frozen and unfrozen earth about twenty inches below the top. After the breach was made in the embankment, large masses of heavy ice came down from the reservoir, which soon broke down the unfinished protection-wall, and carried off nearly the whole embankment. The masonry of the dam and abutment sustained but little injury. Such a flood had not been anticipated, and the watery-way proved insufficient to pass it off. Had the embankment been completed the full width, and the protection-wall carried up to the full height it was intended to carry it, the work might have proved adequate to the emergency. It was determined to fill the gap made by this breach (about 200 feet long) by a structure of hydraulic stone masonry, adapting 180 feet as waste weir. This work presented all the difficulties it was originally intended to avoid, by carrying the work partially into the hill. It was necessary to form an artificial foundation, and carry up a heavy body of masonry in the channel of the river, which, in some parts, had fifteen feet in depth below its ordinary level, subject, in ordinary seasons, to frequent and sudden floods, and affording no means to form another channel for it to pass, until the work could be accomplished.

Had it been admissible to construct the dam with timber, the difficulties would have been far less. But the importance of the object it was designed to secure, and its great height, demanded the most permanent and durable structure that could be made.

The greatest height of the weir of the dam is forty feet above the low-water level, and fifty-five feet above the bed of the river. The width of masonry, at low water line of river, is sixty-one feet.

The form on the lower face commences on a curve, described by a radius of fifty-five feet, and continues to within about ten feet of the top, when a reversed curve, on a radius of ten feet, carries the face over, and meets the back line of the wall. The back line is carried up vertically with occasional off-sets. The main body of the work is laid up of rough stone: the curved face of large and closely-cut stone, with four heavy courses at the bottom, dovetailed together, the joints cut to the line of radius of curve.

Above the masonry an embankment of earth is filled in, and extends to 275 feet in width on the bottom, with a slope of one to five on the up-stream face. The north end of the new weir is terminated by an abutment that rises twelve feet above it. From the toe of the masonry an apron is extended thirty-five feet, composed of hewn timber secured by ties, bolts, and tree-nails, in a very substantial manner, and filled, for sixteen feet from the stone work, with concrete masonry, and the remainder with loose stone, and covered with a course of six inch white elm plank. A second apron is partly made, which is to extend thirty feet further.

At 300 feet below the main dam, a second dam is in progress, which is to be nine feet high, constructed of timber, stone, and gravel, which will set the water back over the apron of the main dam, and form a pool to check the water as it falls on it.

A coffer-dam was constructed in the river to enclose about 120 feet of the work, from which the water was pumped by a steam-engine, and a concrete foundation laid down on a very firm hard pan. The remainder of the foundation was made by sinking timber piers at suitable distances, running parallel with the dam, and filling the spaces between them with concrete masonry. In preparing the specifications for this work, the method and order of prosecuting it, were particularly designed, and from which no material departure has been found necessary; the structure is now nearly completed.

The contractors, Messrs. McCullough, Black, McManus and Hepburn, have evinced a highly commendable energy and ability in its prosecution.

This dam sets the water of the river back five miles, and forms a reservoir of about four hundred acres, and has rendered it necessary to construct several new roads and bridges as a substitute for those covered by the flow; the principal of which is the Somers town turnpike. The grounds lightly flowed on the margin, have been excavated so as to give four and a half feet for the least depth of water. From this reservoir the water flows into the bulkhead, at the upper end of the tunnel, from a level averaging ten feet below the surface.