The two Sumas prairies, and why the Barrowtown floodgates are so important

Image: City of Abbotsford

• Our story on Mt. Baker, and the problems climate change poses for flooding from the Nooksack

• Our story on how to get, and give, help during this crisis

• Our investigation into the failure of the Sumas dike, and the reasons other levees are doomed to fail in the event of an even worse flood from the Fraser

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Sumas Prairie is not actually a single prairie—at least not when one talks about how flood waters interact with the valley bottom.

The prairie is, in fact, divided by a berm that splits it in two, and water in these two prairies behaves very differently. It’s complicated, like so much else about the Sumas valley, but understanding the differences between the two prairies is crucial to understanding the ongoing flood threat, the dike systems, the new tiger dam across Highway 1, and the Barrowtown floodgates. (As of Tuesday afternoon, the floodgates remained open.)

The Sumas dike follows the Sumas River as it wends its way northeast across the prairie. To the south and east of that dike lies flat land and the former bed of Sumas Lake. To the north and west of the dike is the Sumas River, and slightly higher farmland.

The dike exists to keep the two halves of the prairie separate because, as we have witnessed over the last two weeks, when water gets into the lower prairie, it is very hard to get out. Ideally, by routing the flood waters away from the lower prairie, the dike allows the Sumas River to flow downhill, through the Barrowtown floodgates and into the Fraser River via the Vedder Canal. That is what normally happens to water in the Sumas River basin during and after large storms.

Some water naturally collects in the eastern, lower prairie. And because the bed of Sumas Lake lies below the level of nearby watercourses, it collects in a canal and is normally pumped out through the Barrowtown Pump Station.

The key is keeping the water out of that lower prairie, which stretches all the way into Yarrow, in Chilliwack. Flooding in the higher prairie can upset lives and plans, but it also can drain relatively quickly. That’s not the case when water collects in the lower prairie.

What is important to recognize is that once the flood water is in the lower prairie, the floodgates and gravity won’t help it escape to the Fraser. In that situation, it must be pumped up and out at Barrowtown Pump Station. That, though, is less efficient than using the flood gates to harness the power of gravity. When open, the floodgates can allow 3.5 million gallons of water per minute to pass through them. The Barrowtown pumps—some of the largest in all of North America—handle one-seventh that total.

Those floodgates are key, and one reason why Abbotsford Mayor Henry Braun was so worried on Sunday when they needed to be closed. Gravity can only do its thing and drain the upper prairie when the Sumas River is higher than the Fraser River and Vedder Canal. When and if those watercourses rise, the floodgates must close. Otherwise, water from those places would cause the river to begin running backwards into the upper prairie.

With the floodgates closed, however, there is nowhere for the water to go in the higher, western section of prairie. So it builds up. That’s what happened in the first flood: water poured north from the Nooksack, but for days, the Fraser was too high to allow it out through the floodgates. That water pooled, and eventually overtopped the dikes, causing two large breaches that then allowed water into the lower Sumas Prairie.

On Sunday, Mayor Henry Braun called a repeat of that situation his “doomsday scenario.”

The dikes, along with the new Highway 1 tiger dam erected over the weekend at the Sumas Canal are intended to keep the two prairies firmly separated, so that even if water starts to build up, it stays in the upper prairie, where it can be drained by the flood gates. Without the dam, the worry was that water would essentially take the highway’s bridge from from the upper prairie into the lower prairie. So the new dam aims to shut down that handy little detour, and keep flood traffic heading toward those floodgates.

The hope will be that those gates will allow water to continue flowing into the Fraser and that, even if water does start to rise in the western prairie, it won’t get as high as it did during the first storm.

That’s the hope, anyways.