Mille Roche Power House

5 min read

Power House
35 – 75 ffw
Lost Villages
N 45 01.300 W 74 49.860

The Mille Roches Powerhouse, a fascinating piece of Canadian engineering history, lies submerged beneath Lake St. Lawrence. Before its submersion due to the St. Lawrence Seaway project in 1958, it played a crucial role in the region’s industrial growth, providing electricity to the Cornwall Canal and surrounding areas.

A similar powerhouse exists on Victoria Island in Ottawa, predating Mille Roches by a few years. Remarkably, it remains operational, feeding power into the local grid. An acquaintance of Tom’s had connections with someone working there, which allowed Tom to tour this facility two decades ago. Coincidentally, the visit coincided with a maintenance period that had drained the headpond, allowing them to observe inner workings that explained some of the mysteries surrounding the Mille Roches Powerhouse.

Both the Ottawa and Mille Roches powerhouses feature horizontal turbines and generators. However, unlike the Ottawa counterpart, Mille Roches is equipped with headgates to shut off water flow to the turbines. This feature, divers discovered, also restricts access to the wheelpits and the turbines themselves. The trashgates in front of the headgates are still in place, along with the large winches and gears used to lift them. The powerhouse originally housed four wheelpits, with only two turbines installed. A third wheelpit contained an “exciter,” essential for charging the generator. The fourth wheelpit, partially closed off, offers a tight fit for scuba gear but rewards divers with an exhilarating exploration experience. An unusual square hole in the large dome covering this wheelpit allows light to penetrate, creating a surreal underwater ambiance. Diving inside this wheelpit is one of Tom’s fondest memories.

It’s said that Thomas Edison himself attended the grand opening of the powerhouse, ceremoniously “throwing the switch” to start generating power. The Cornwall Canal was the first customer, with lights running along its eleven-mile length from the Cornwall Harbour locks to the western entrance at Lock 21. As local factories upgraded their machinery, more customers appeared, boosting the region’s industrial growth.

The late Mr. Archie Eastman, an operator at the powerhouse until its closure, shared intriguing details about controlling electricity output. Primitive devices inside the control room generated significant heat, an unavoidable byproduct of maintaining a steady electrical flow despite fluctuations in Bergin Lake, the headpond. Some remnants of these early devices were discarded into the tailrace.

Different photos capture the powerhouse’s history, including a large metal vent once atop the tower. This metal appendage now rests at the bottom in front of the trashgates. The metal domes covering the wheelpits with turbines are buried under debris and bricks from demolition.

A rare color photograph shows the tailrace behind the powerhouse, flowing down the last remaining stretch of the original north channel of the St. Lawrence River at Mille Roches. Today, 75 feet of water covers this site.

The photograph below depicts the powerhouse during demolition. Most trees were cut down, and the north channel was dried for constructing the new power dam. The remains of the generators, circular pieces salvaged for their copper, lay on the ground to the right of the building. Only two turbine shafts and some operating hardware, like the reciprocating shafts and gears that controlled turbine blade covers, remain on the generating room wall.

Nick Batea 1989c — Sketch by Nick Baets 1989, donated toSave Ontario Shipwrecks

A wonderful sketch by Mr. Nick Baets, an avid Ottawa scuba diver, captures what remains of the powerhouse. He included the entire flotilla of boats and dive platforms from a great dive social organized by a local chapter of Save Ontario Shipwrecks two decades ago. Divers surveyed most of the site, documenting various structures and artifacts on multiple levels.

The site is divided into three levels at roughly 35, 55, and 75 feet of depth, depending on Lake St. Lawrence’s water level. There are far more bricks strewn about than shown in the drawing, but most large equipment is easily accessible. A favorite spot is inside the left wheelpit, where divers can see the headgate. The winches above the headgates offer much to explore, and divers can drop below the wooden walkway between the closed headgates and the trashgates, which create unique lighting through the bars onto schools of bass and perch hiding from larger predators.

The middle level, where the generating room was located, and the stone arches below, where water was discharged, consist of solid concrete and stone. They should withstand time and offer visiting divers a glimpse of the original lost villages for many years.

Become a dedicated fan of this site and plans to return next year for at least a two-tank dive. If divers wish to explore any of the holes, a bright light is essential. The 21-watt LED Lightmonkey barely illuminated half of the stone discharge area at 75 feet. Visibility was about 25 to 30 feet (10m), and the current was strong, with water temperature around 70°F (21°C).

They started by descending the line (remember to conserve air by using your arms, not your legs) and began at the winches for the sluices (or headgates, as they appear to be). Heading with the current, perpendicular to the dam, they proceeded to the next level, about 20 feet (6 meters) down. They didn’t spend much time on the flywheel backs, as they planned to see them on the return.

At about 40 feet (12 meters), they dropped to the lower level with the huge arches (tailraces?). Entering the right-hand arch, they were amazed by its size. They ventured to the back, where giant tubes allowed water to leave the powerhouse. After exploring the arches, they returned to the second level to see the two water turbines. On the left, there’s a room where a turbine was removed. With good buoyancy, one can enter this room, which contains very silty mud. The walls are intriguing. After exploring the flywheel/turbine ends, they returned to the top level.

A delightful surprise awaited them here. Although unseen in pictures, the domes above the turbine rooms had been slid back, allowing entry into the room. Did they mention these rooms are huge? Maintaining buoyancy is crucial, as there’s about five feet of silty mud at the bottom. The turbines are fascinating; Tom spent at least 10 minutes navigating around them. Some walls are partially wooden and in excellent condition, considering they’ve been underwater for almost 60 years. More time is needed in these rooms to understand the water’s flow.

Upon exiting the rooms, they moved to the powerhouse’s front, where a grate prevents large debris from entering the gates. They descended between the grate and the forward wall, traveling from one side and back. It was a remarkable experience, but the dive ended too soon, and they surfaced.

The creation of the St. Lawrence Seaway in 1958 increased water depth, enabling larger ships to access the Great Lakes. This resulted in the submersion of numerous villages, roadways, locks, powerhouses, and canals, now popular dive sites. One such site is near the former Maple Grove in Cornwall, Eastern Ontario, Canada. Divers can explore the Mille Roches Powerhouse, which offers a maximum depth of 17 meters and visibility of about 4 meters, making it suitable for beginners. For more information and a detailed site description, please visit the official website of Save Ontario Shipwrecks.