The South Fork Reservoir had originally been planned and built to supply the Pennsylvania Canal with water during the dry season. Although the State Works possessed a dam across the Conemaugh east of the basin and a feeder dam for the basin on the Stonycreek, additional supplies of water were needed to maintain the level of the canal from Johnstown to Blairsville. In 1834 the canal commissioners called the attention of the legislature to the probable failure of the rivers to furnish a sufficient supply, and sought authority and funds to conduct surveys for a reservoir site for the western division somewhere east of Johnstown.
For a period of three years the engineers coursed the hillsides and gullies of Allegheny Mountain for a possible site. As a guide for their selection the engineers possessed earlier surveys of the Conemaugh, such as the survey of 1791 by the Society for Promoting the Improvement of Roads and Navigation.51 The federal engineers’ survey and report of 1822 had recommended a dam at the headwaters of South Fork Creek to furnish water for a western canal.52 The very rapid descent of the Stonycreek made a dam on it impracticable in times of spring floods.53 The South Fork site was the only one that offered the necessary requirements of the state engineers. Draining an area of fifty square miles at an elevation of four hundred feet above the canal at Johnstown, a reservoir on the South Fork would furnish a sure supply of a large amount of water for the canal.54 In 1839 the canal commissioners reported their surveys to the legislature; recommended the damming of South Fork Creek:
There can be no doubt of the sufficiency of this stream to fill a reservoir of any desired capacity. It was carefully gauged in September, after one day’s rain, and found to discharge in 24 hours, sixty million cubic feet of water. At the same time, there were flood marks along the stream two feet higher; it would be a moderate estimate to suppose a flood of two additional feet would discharge three times as much water; equal to one hundred and sixty millions of cubic feet in twenty-four hours. 55
In the same report the commissioners sketched a plan for a dam that would cost $188,000. A wall 850 feet long and 62 feet high would impound 480,000,000 cubic feet of water. Such a reservoir would discharge 3,500 cubic feet per second into the Conemaugh River. 56 The Commissioners sought an appropriation to start work on their new dam; the Legislature allotted $70,000. The commissioners then drew up specifications for the South Fork Reservoir. The wall of the reservoir was to be ten feet higher than water level. The wall was to contain a core of well puddle clay supported on both sides by stone and gravel layers. The outer surfaces of the dam were to be rip-rapped with stone; the outer surface to be covered with heavier slabs than the water wall. The slope of the outer wall was set at 1-1/2 to 1: the inner slope 2 to 1.
The wall of the reservoir, as originally planned and built, was well made and safe. The puddle core was twenty feet wide and extended four feet above the water line. The outer wall’s stone layer was twenty feet thick at the base and four feet thick at the top. Between the puddle core and the rip-rap was a layer of native slate broken into a four-inch size.57 The western end of the wall was anchored to the hillsides by a stone wall twenty feet high and buttressed every twenty feet through the base.
The control mechanism for the dam consisted of a sluice gate whose pipes, covered by a rock culvert, extended entirely through the base of the embankment. A valve tower, inside the reservoir, was used to regulate the discharge of water through the five twenty-four inch iron pipes. At the eastern end of the dam a spillway, seventy-two feet wide and ten feet deep, was cut through to the natural rock of the hillside.58
With $70,000 available for the new project, the commissioners awarded contracts to James K. Moorehead of Pittsburgh and H. E. Parker of Williamsport on January 31, 1840, for the construction of the wall. Building continued until November 1841, when funds were exhausted. In their Report for 1840, the commissioners reported that all the work below the sluice pipes had been completed. 59 In the following year, they reported that there was a depth of sixty-two feet of water in the reservoir which then covered 420 acres of land and impounded 480,000,000 cubic feet of water.60 Accompanying the report of progress was a revised estimate of the total cost of the dam. The commissioners asked for $188,000 to complete the reservoir. The legislature, hard pressed for funds, was in no mood to grant such a sum; but did vote $50,000 to pay off the contractors for all work to be completed by May 1, 1841. Not until 1846 did the legislature assign more money to the project. In this year a grant of $20,000 was made; but due to the disastrous floods of 1846 the canal commissioners used the reservoir money for repairs to the State Works. No further money was granted until 1850 when $45,000 was voted. The commissioners delayed so long in resuming construction that this sum reverted to the state’s treasury. In 1851, when the legislature granted $45,000, the commissioners quickly resumed construction. The following year, an additional $55,000 enabled the commissioners to complete the South Fork Reservoir. On June 10, 1852, the sluice gates were closed; by September, forty feet of water was behind the embankment. In the fall, water from the dam flowed to the canal for the first time. The dam, at completion, had cost $166,647.50.61 The area flooded in 1853 was 424 acres.62 Behind the reservoir’s embankment, 931 feet long and 72 feet high, with a road along the top, 480,000,000 cubic feet of water were impounded.63 William N. Morris, the state engineer and the canal commissioners had constructed the largest earthen dam in the world. The water in the reservoir was sufficient to maintain five feet of water in a canal, 560 miles long.64
Excerpt from Nathan Daniel Shappee, History of Johnstown, Unpublished doctoral dissertation, University of Pittsburgh, 1940. Pages 257, 260–261.
