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The Story of the Building of the Hogs Back Dam by Ken W. Watson "I was standing on top of it [the dam] with forty men, employed in trying to stop the leak, when I felt a motion like an earthquake, and instantly ordered the men to run, the Stones falling from under my feet as I moved off.” [Price p.108]. These are the words of Lieutenant-Colonel John By, describing the third failure of the Hogs Back Dam on April 3, 1829. The story of the Hogs Back dam starts with the decision to put the entrance of the Rideau Canal in the location it is today (the Ottawa Locks – Entrance Valley). By was attempting to avoid the hard bedrock excavation that would have been required if they had followed the route proposed by Samuel Clowes in 1823/24, which had the canal’s entrance at Rideau Falls. His final route (which surveyor John MacTaggart worked to figure out – see the story of Christmas 1826), avoided any significant bedrock excavation. But it did mean that a big dam was needed at Hogs Back, which in the pre-canal era was known as Three Rock Rapids. Although the rapids themselves only had a drop of 6 feet (there were no falls here in the pre-canal era – see Jebb’s map in the Burritts family story), the dam had to do two things. It had to flood the Rideau River all the way to Black Rapids in order to put a navigation depth of water over the lower sill of the lock at that location. It also had to put water over the upper sill at Hogs Back, the entrance to the canal cut to the Ottawa locks. To do this, it had to raise the water of the river in this location by about 41 feet.
A problem with building a slackwater canal system (system where dams transform the flow of a river into still water), is that the dams have to be built across the width of the river. What do you do with the river water while building the dam? At the time of the building of the Rideau Canal, there were essentially three choices. If the dam was small enough, you could build a waste weir (controllable water bypass) and then just build the dam in the river, letting the river water flow through your weir. This was difficult to do with larger dams and higher water flows, these necessitated either building sluiceways in the dam itself (this technique was used for the dam at Jones Falls) or using a coffer dam (a temporary dam upstream from your work area) to divert water around the construction of the dam in the initial phases, and then using the coffer dam to divert the water into your waste weir while you complete the dam. This coffer dam method was used at Hogs Back. Another issue at the time was the height of the dam based on its intended use. By’s original thoughts were to follow European convention and construct overflow dams. That is, if you wanted to raise the water 10 feet, you built a 10 foot high dam and let the water flow over it. The original design for Hogs Back was for an overflow dam, over forty feet in height. After By’s first experience with the Rideau River in spring flood he quickly changed his plans, raising the height of many of his dams so that they were no longer overflow dams, and incorporating a waste water channel with a weir (water control mechanism). In about June 1827, after his contract for the work had been signed, Fenlon started the stone dam construction on the east bank of the river. He opened a quarry in this location and ran a “rail road” to the dam location. A coffer dam was built upstream to divert the water of the Rideau River to the west side. This essentially cut the width of the river in half, allowing Fenlon to construct half the dam in the now relatively dry east side. The coffer dam was made up in part of a jetty extending from the east bank, consisting of stones,
After completing the preparatory work, Fenlon started to build the stone dam. Work progressed so well that by the fall of 1827, Fenlon thought he was close enough to be able to close up his dam. To do this, he excavated a waste weir channel in the east bank, this is where the water of the Rideau River was to flow once his dam was raised. The bottom of his waste weir channel was about 27 feet above the bed of the Rideau River. Once the river was raised to that height, it would flow through the weir. The main portion of his dam had been raised to 37 feet and work was progressing well on closing up the area between the completed portion of the dam and the west bank, when, in February 1828, a sudden unexpected rise of water washed the west half away, essentially returning it to the state it had been in early fall 1827 (only the east portion of the dam remained). Learning from experience, a slightly modified plan was executed. The concept of turning the water into the waste weir was sound if it could be effectively accomplished. The waste weir was excavated down another four feet and was made forty feet wide. The largest timber that could be found, pine and hemlock, fifty to sixty feet long were used in the mill dam. Even with those lengths, the mill dam could only be raised two feet above the bottom of the newly excavated weir. The Rideau River proved more than a match for these engineering efforts. On April 1, 1828, spring flood water both topped the mill dam and eroded out the bank of the river. These flood waters in turn severely damaged the stone dam. Work began again on repairing the dam. However, Fenlon was a bit fed-up and begged to be released from his contract. He wrote a letter to Colonel By on June 18, 1828, in which he stated: “I find that I cannot possibly continue the Work at the prices that I am at present getting according to my Contract and I am the looser to a great amount on what I have already done. My humble prayer at this time, is, that Government would take the job and release me from all claims on the Contract. I trust I shall be allowed an estimate on what I have done in preparation for carrying on the work, and my losses I submit to the Consideration and discretion of the Commanding Officer.” [Price pp.103-104] It appears that it was not until the fall of 1828 that Fenlon was released from his contract. It was noted by Colonel By and the on-site overseer, Capt. J.C. Victor of the Royal Engineers, that Mr. P. Wright and Sons had taken over part of Fenlon’s contract, a new contract being awarded to Wright on November 1, 1828, and in addition, that masons of the Royal Sappers and Miners were employed in building the arch keywork of the dam. By noted that “it will require great exertion during the whole of this winter to raise the arch Key work to a sufficient height to resist the spring floods;…” The dam stood about 22 feet high when Fenlon was released from his contract in November 1828. The Sappers and Miners and a crew of upwards of 300 labourers took over the job of completing the dam. Wright’s crews worked on the coffer dam. With the experience of two dam failures behind them, they widened the waste weir channel to 60 feet, in order to provide lots of capacity (so they thought) to divert spring flood waters. According to By, the dam had been raised 60 feet by late March 1829 (50 feet according to Denison). A height of 60 feet was 15 feet above the required height of 45 feet, but By wanted the dam bigger and stronger. The base (stone, core, apron) was over 200 feet thick. The water had been raised the intended 41 feet, there was now flat water all the way to Black Rapids. By March 28, 1829 the Rideau River was in full flood, with the water rapidly rising in front of the dam. According to By it was on that day that the dam started to leak and efforts were started to try to stem the leakage (Denison says the leak started on the April 2 and efforts to stem the leak started on that day). On April 2, large scale slumping occurred in the lower part of the centre section of the dam. On April 3 the dam failed, the flood waters washing much of it away. The quote from Colonel By at the beginning of this article describes what happened when the entire dam failed. The failure was due in part to inexperience with cold weather engineering.
The slumping that occurred on April 2 was the unfrozen portion slumping down below the frozen earth, allowing water, now under extreme hydraulic pressure, to penetrate to the clay puddle and start washing it away. The dam had sprung a leak. When this was noticed, attempts were immediately made to stem the leak by throwing everything they could find (brush, timber, clay, earth) into the source of the leak. The idea (and this had previously succeeded with a similar leak in the dam at Smiths Falls) was that this material would temporarily stem the flow, allowing an effective repair to be made. They also had men clearing out debris from the waste channel, trying to increase this bypass flow of water. But these efforts were far too late. The “earthquake” By described was the leak fully breaching the keywork of the dam. Now the entire flow of the Rideau River was going through the dam. The reason there were no lives lost or even injuries is that the frozen mass of earth held for some time, creating an arch above the rushing torrent. Denison reported that the arch of frozen earth stood for five minutes before the running water won out, and the arch collapsed into the torrent. By stated that “The force of the water was such that stones of two or three tons weight were tossed about as if they had been blocks of wood, and that the frozen earth was carried over the Rideau Falls, a distance of between five and six miles.” [Price p. 109] By was to make two changes to his construction plans for the dam. Wright’s cribbed coffer dam had suffered very little damage. So By planned to use this technique to construct a “temporary” dam made of cribbed timber filled with stone. The second change was the necessity to do this when the water flow allowed and the temperatures were above freezing – so this new dam could only be worked on from the beginning of July to the end of November. In his April 23, 1829 report on the failure of the dam, he still appeared optimistic that a stone dam could be raised “at a future period.” Denison says that any plans for a stone dam were abandoned as work progressed on the timber crib dam. Part of the original keywork dam still stood on the east side, but the entire west side of the river was open. The new timber dam construction was a model of simplicity. Timber cribs were laid on the bedrock of the river. As the first course of timber was laid, it was filled with large broken stones to resist the flow of the river. Once this base was secure, they laid another course of timber on top of this, creating an open timber framework through which the river flowed. The timber crib work was raised to the required height of the dam.
By November 30, 1829 the dam had been completed to the point where water was running through the waste weir. Although some minor slumping was noted, there was no failure of the huge mass of earth, timber and stone. It wasn’t as elegant as a stone arch dam, but it worked. The failures of the Hogs Back dam weighed heavily on By’s mind. In a letter written on December 31, 1829 by By to his superior, General Mann, By stated “… I have the honour to report, that the dam at the Hog’s Back is nearly completed, and answers the desired object in every respect, having raised the Rideau River to the required height of forty-five feet, and thrown back six feet depth of water into the lock at Black Rapids, which proves my original levels at this place to be correct, and also the practicability of my project, which, when the dam gave way last April, was doubted by many, and to this annoyance I attribute the serious illness with which I was afflicted in April last.” [Price p. 114] When the new dam and weir survived the spring floods of 1830 we can be sure that Colonel By and his men all let out a great sigh of relief. 
Sources: "Rideau Dams", by Lieutenant W. Denison, in "Papers on Subjects Connected with the Duties of the Corps of Royal Engineers", London, vol. 2, 1838, pp. 114-121. "Account of the Causes which led to the Construction of the Rideau Canal, connecting the Waters of Lake Ontario and the Ottawa; the Nature of the Communication prior to 1827; and a Description of the Works by means of which it is converted into a Steam-boat Navigation", by Lieutenant Edward C. Frome, in "Papers on Subjects Connected with the Duties of the Corps of Royal Engineers", London, vol. 1, 1837, pp. 73-102. "Construction History of the Rideau Canal," by Karen Price, Manuscript Report 193, Parks Canada, Ottawa, 1976.
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