Dams didn’t release too much water, officials say

The organization that oversees Ottawa River dams says it did not add to the flooding problem in Ottawa and Gatineau by letting too much water through its dams farther upstream.

Facebook reports have been circulating with pictures of low water on the river near Mattawa, about 300 kilometres upstream, and claiming this contributed to flooding in Ottawa and Gatineau.

But the Ottawa River Regulation Planning Board, which oversees the dams, says there was nowhere left to hold the torrential rain on May 1.

The board represents different levels of government and utilities on both sides of the river.

The problem is that dams upstream from Ottawa aren’t designed to hold back water to form a significant reservoir, said Michael Sarich, the board’s senior regulation engineer.

Yes, he said Thursday, it’s true that water near Mattawa was low. But that doesn’t mean that the closest dam — at Rapides des Joachims (known as Swisha on the Ontario side) could have held back more water. The dam is owned by Ontario Power Generation, but the Ottawa River board tells OPG how much water to let through.

If the Des Joachims dam allows water to build too high instead of releasing it downstream, that would flood Mattawa, he said.

“This is a question that we get every year. It’s not something special this year,” he said.

Legislation requires that the dams not make any flooding worse than what would happen under natural conditions, he said. That means that from Temiscaming through to Montreal, “as flows increase, dam operators continue to open their gates and essentially pass all flow that is coming.”

Des Joachims is “tricky” because when water rises at the dam, it can block more water from coming downstream behind it. Engineers call it a “constriction point” on the river.

The water is let down every year in late winter to prevent a backup during the spring flood, he said.

“That water, if you were to maintain your normal summer level under highflow conditions, would allow water to back up (upstream).

“It’s very simple in that water runs downhill. So if the bottom of the hill (water at the dam) is higher, it makes the top of the hill (the river near Mattawa) higher too.

“It would prevent water from coming down and it would flood Mattawa.”

When heavy rain came across the region in early May, operators had to let water through the big dam to avoid a backup. That led to the social media complaints from people saying they should have held back the water to protect Gatineau and others downstream.

The dam is owned by Ontario Power Generation (not Hydro One, as reported in many social media posts) and regulated by the Ottawa River Regulation Planning Board.