Rivière St. Pierre, Part V: Ovalflow

Following Rivière St Pierre as it makes its way through the village of Rockfield and (sometimes) into the Lachine Canal

Inside the Rockfield overflow sewer in Lachine.

Rivière St. Pierre, Part V: Ovalflow

Following Rivière St Pierre as it makes its way through the village of Rockfield and (sometimes) into the Lachine Canal

Picking up from where we last left off, this stretch takes us through the inside of the Cote-St-Luc collector sewer at the northern edge of Lachine. From here it snakes its way southwards towards the Lachine canal, never straying too far from the original course of Riviere St. Pierre. Approximate round-travel distance: 4 kilometers.

I never look forward to having to cover larger distances inside sewers here in Montreal. The depth will vary, but 2-3 feet tends to be the norm. That might not seem like a lot, but when you factor in the speed at which it’s flowing and the amount of gear these types of trips can entail, it doesn’t take long before it starts to feel like a solid cardiovascular workout. This is especially true when you’re walking against the flow.

Here’s a general overview showing the CSL collector (the dark red line), the estimated path of Riviere St. Pierre and a few of the features described below.

For the first kilometer and a half, this portion of CSL collector is pretty straightforward: 8×10 feet in diameter with a shape that’s typical of concrete sewers built in Montreal between the 1920s and 1970s. A smooth horseshoe-shaped arch covers a course aggregate floor that that slopes towards the edges. During dry weather, the edges of these types of sewers are usually dry. City plans define this section the “sidewalk”; but that term’s a bit misleading. Due to the angle, it’s actually terrible to have to walk on.

Because of the broader arc of the sewer and for some of the features described below, I decided to name this length of the CSL Collector “Ovalflow.”

It’s unclear when the sewer replaced this portion of Riviere St. Pierre. While bits and pieces seem to have been covered as early as the 1930s, I suspect everything was eventually reconstructed during the mid-1960s when the entire area’s sewers were integrated into a much broader system.

By the time the sewer reaches the area of Lachine known as Rockfield, things start to get a bit more interesting.

At this point the CSL collector officially comes to an end inside a rather unusual chamber containing a number of interesting features. As seen in the photo above, on one side of the channel are four cells. All the water flows into the cell closest to the camera at which point it falls down a sinkhole into an even larger sewer- the appropriately named St-Pierre Collector, which we’ll get to in future entries.

The other three cells contain a metal cylindrical object suspended directly above a three-foot wide hole, one of which is entirely blocked up with sludge. Above these are a number of counterweights and a mechanism (shown here) likely designed to lower each cylinder so that it covers the hole. Since each hole seems to lead to the same sewer further below, I’m not sure what purpose these things actually serve. A water monitoring device (or flowmeter) sits high in one corner of the chamber, but it’s unknown if it has any connection to the large plugs or if it simply sends an alert whenever water levels start to rise.


Also found in this chamber is the beginning of a rectangular shaped conduit situated roughly eight feet above the main channel of the sewer. The principle here is simple: when the water inside the sewer reaches a certain height, the surplus flow is diverted through this tunnel and out into the open water- in this case, the Lachine Canal.

This is one of the drawbacks of a combined sewer system. Most of the time everything gets sent off to the treatment plant as intended, but during heavy rainstorms, the system can become overburdened to the point where sewage spills out into the open water. Fortunately, these events only seem to happen a few times a year, but they’re still a source of significant water contamination. If you’ve been warned not to swim in the Lachine Canal (or elsewhere on the island), combined sewer overflow events are likely the reason why.

This eleven by five foot high conduit (known as the Rockfield Overflow) runs for a quarter of a kilometer, but because the outfall at the canal is submerged, one can only travel about two thirds that distance. Any further and you’re soon up to your chest in canal water.

At this point your only choice is to turn around.