Completed in 1822, Fairmount Dam cut diagonally across the river, channeled water into the Waterworks, and acted as a spillway. The Schuylkill is a tidal river, so it also prevented brackish water in the Delaware River from mixing with the fresh water. Above it, the dam created the "Schuylkill Pond", several miles of tranquil water used for recreation. The original dam was replaced in 1928. SOURCE Although repairs and additions to Fairmount Dam have been undertaken in the years since its construction in 1819-1821, the original cribs have remained as placed, with the added support constructions before and behind. A recent article about the dam reopening and being renovated/updated can be found Here.
The decision in 1819 to construct a dam at Fairmount presented a challenge that had not been undertaken prior to that date. The river was wide and the current very fast. The Schuylkill River before the construction of the Fairmount Dam in 1819-1921 was a fast-flowing river subject to periodic freshets caused by rainfall over its extensive drainage area. After the dam was constructed, the river has remained fast-flowing, but the current is not as easily seen because of the six-mile pond created above the dam which gives the false impression of quietness, but is much respected by the rowing clubs and canoeists using the river.
When the Watering Committee made plans to convert the Fairmount Water Works to a water-power pumping station, it was necessary to arrange for a dam to be constructed across the river near the site. The Schuylkill Navigation Company had been chartered in 1815 and was in the process of making the Schuylkill a navigable waterway as far as the coal fields up the river. This involved a series of canals, locks, and dams. In 1819 the Navigation Company was in the process of completing the Flat Rock Dam at Manayunk, and the contractor was Ariel Cooley of Chicopee, Massachusetts.
The city purchased the water power rights at the Falls of the Schuylkill—the natural fall line of the river—from Josiah White and Joseph Gillingham for $150,000. Construction of Fairmount Dam would back the water up for six miles and cover the falls as well as inundate some of the islands in the river. Under the agreement, in addition to the dam, the Watering Committee also had to build a canal and locks for the Schuylkill Navigation, and these were to be turned over to that entity upon completion.
Proposals for the dam were received, and a contract was signed with Ariel Cooley to build the dam, the canal and locks, and also the head arches and mill race. He started work on April 18, 1819.
Most of the river bed was rock, but the depth varied. The dam was designed as an over-fall and was 1204 feet long, built of cribs of large hemlock logs filled with stone and planked over with oak plank, running up and down stream, which were sunk in the river and fastened to each other and to the rock bed. At the eastern end there was a stone pier joining it to a mound dam 270 feet long and 150 feet at the base, 12 feet at the top, which was 15 feet higher than the over-fall dam. The mound dam was fashioned of earth and quarry spalls, and was built on the part of the river bed that had 11 feet of mud above the rock bottom. It was calculated that it would be too difficult to anchor cribs in that location. Beyond the mound dam were three head arches 104 feet overall, situated at the entrance to the millrace which had to be cut out of solid rock.
Cooley's dam crossed the river diagonally, upstream, turned at an angle near the western shore, providing a large over-fall, and joined the head pier of the canal guard locks at a right angle. This design helped protect the dam from injury from ice and freshets, both of which were a danger on the Schuylkill. The last crib of the dam was put in place in June 25, 1821, and the first water flowed over it July 23, 1821. 1 The millrace, parallel to the river on the east bank, was blasted out of solid rock to a total width of 140 feet. It was 419 feet long, 90 feet wide, and from 16 to 60 feet deep, allowing continual passage of 408 square feet of water to the forebays of the water wheels in the Mill House.
During and after construction, there were questions about the dam's structure and reliability, and it was repaired and rebuilt, but it never gave way, although during some of the repairs it was found amazing that the structure still held. In 1842-1843 the dam was rebuilt from low tide up, and in 1865 a new crib was sunk in front of the old dam. Because of poor workmanship, it was necessary to place new cribs in front of the 1865 cribs, and a new dam was built on top in 1872. Through all these changes, the over-fall of the dam has moved 38 feet beyond the original line of the dam and a new pier had to be constructed to link it with the mound dam. In the more recent past the U.S. Army Corps of Engineers has taken over the care of Fairmount Dam, and has rebuilt the structure with concrete.
The industrial archeology of Fairmount Dam is under water, but its presence cannot be denied. The canal locks on the west bank of the river were destroyed in the 1950s with the construction of the West River Drive and the Schuylkill Expressway. However, a remnant of the wall of the guard lock remains as part of the fish ladder constructed at the site in c.1980. 2 It is possible to consider the Schuylkill Pond created by the dam as a feature of importance to industrial archeologists. Few Philadelphians realize the vital part played by this 1821 structure in providing the city with a recreational facility of unsurpassable beauty and which was so essential a part of the creation of Fairmount Park but which was constructed for purely utilitarian purposes. SOURCE