The construction of "The Great Stone Dam", is perhaps the greatest contributing factor in developing Lawrence as one of the leading manufacturing cities of the country, was begun in the summer of 1845 and completed three years later.

     At the time of  building it was the most massive structure of the kind in the country and it remains, after more than three score and ten years, as permanent and complete as when first imbedded upon the solid rock foundation.

     Within a few months of organization of the Essex Company, July 5, 1845, the contract for the construction work was left to Gilmore & Carpenter.  Excavations commenced August 1, 1845.  The first stone was laid in the foundation line of headers at a point near the center of the river by John A. Carpenter of the firm of contractors on September 1845 and three years afterward, on September 19, 1848, completed structure was laid at a point above the first bed stone, under the direction of the same contractor.  The engineer in charge of the construction was Charles H. Bigelow, a captain of engineers in the United States Army.  Under his supervision the north canal was also constructed.

     The dam is constructed of huge granite blocks, laid in hydraulic cement firmly embedded and bolted upon the river rock bed.  The thickness at the base is 35 feet, narrowed gradually to about 13 feet below the crest stone.  The greatest height of masonry is 342 feet and the average plunge of water is 25-26 feet without flashboards.  The masonry, including wings extending inland is 1629 feet in length.  The overflow of water is 900 feet wide from wing to wing, the crest line curving slightly upstream.  A solid filling of earth, backing the masonry and sloping back, one foot in six, protects the structure.  The south wing wall is 324 feet long and the north wing 405 feet.  The original cost of the structure , at that time of building when prices of labor and material ruled low, was about $250,000.

     It is conceded to this day that the dam and guard locks were an advance upon engineering methods at the time.  No rebuilding or special repair has been needed as no weakness or defect has been apparent since its completing.  It stood the test of the flood of 1852, the most destructive on record for the Merrimack River, when the old toll house, part of the Falls bridge and the fishway went into the swirl of water, ice, logs and debris. 

Note:  The dam has survived the flood of 1938 and the flood of 2006 and 2007.

Dam Update 2007