Why we flood: Capricious climate complicates river regulation

The Carillon dam is the Ottawa River's final drain.

Water from every one of the river's many tributaries and reservoirsevery drop from its vast watershed eventually flows through the hydroelectric power station's gates, about 130 kilometres east of the capital.

On April 30, whenthe river reached this spring's peak, a record 9.2 million litres surged through the dam each second, enough to fill nearly four Olympic-sizeswimming pools. That's more water than cascades over Niagara Falls.

It was a similar scene in 2017, when just over nine million litres per second crashed through the dam at the height of that spring's flood.

Both that spring and this one, shoreline communities were deluged, homesdestroyed and residents left picking up the pieces.

But while the 2017 floods were blamed largely on unusually heavy rain that April and May, the seeds of this year's floods were planted last November.

2019 all about the snow

This past winter was a record-breaker for snowfall in and around Ottawa.

More than one metre of snow fell on the capital in January alone.

The dress rehearsal has taken place, and it wouldn't surprise me to see these situations happening more frequently.- David Phillips, Environment Canada

There were 155 straight days with snow on the ground,and eventually, it all had to melt somewhere.

Michael Sarich, an engineer with the Ottawa River Regulation Planning Board (ORRPB), said the unusually thick snowpack certainly contributed to this spring's conditions, but it wasn't the only factor.

"We've had snow years that are bigger than this 2008 would have been considered a higher snowpack in terms of water volume but it wasn't a record-setting flood year by any means," he said.

The problem with last winter's snow was its staying power.

"It didn't disappear until April 12. Every time you looked out the window in Ottawa you saw snow on the ground," said David Phillips, senior climatologist for Environment Canada.

In the northern part of the watershed, the snowpack was even thicker, and lingeredeven longer.

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It was also a cold winter.

According to Phillips, monthly average temperatures in and around Ottawa were consistently below normal, leaving the ground frozen longer and the snow on top of it with nowhere to go once it started melting.

"The ground got progressively colder and therefore when melting did occur, it ran off at the surface," he said.

Sarich said thathigh volume of surface runoff combined with plenty of rain, which, while not as heavy or consistent as in 2017, still made for a perilous mixture.

"It's a combination of those three factors,precipitation, temperature, and snowpack that eventually determines your total volume."

Regulator limited

Sarich said since 2017, the ORRPB has worked toimprove how itcommunicates withmunicipalities.

When it comes to actually managing water levels, however, Sarich said the regulator is limited.

"People associate regulation with that they're living on a system that is fully regulated or perhaps even highly regulated. None of the above is true," he said.

In fact, two-thirds of the river's length is effectivelyuncontrolled,with no reservoirs or dams to manage the flow.

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The primary reservoirs are in the watershed's northern reaches. They were lowered this spring below 2017 levels to make way for the anticipated melt, but quickly refilled and had to be emptied downstream.

"The size of our bucket stays the same in terms of being empty every spring. That's standard operational procedure," Sarich said.

What the regulator can do, Sarich said, is coordinate with reservoir operators alongthe river to ensure everyone is working with the same data, but eventually those dams must be opened and the river must flow.

Could have been worse

Despite their limitations, the reservoirs upstream can help prevent disaster.

The ORRPB estimated that in both 2017 and 2019, river levels downstream would have been 20 per cent higher without them.

"Levels would have been so much higher without these reservoirs and without the proper application of storage," Sarich said.

Phillips agrees that while both floods hit communities along the river hard, it could have been much worseif there's ever a year where 2017-level rainfall meets 2019-level snowpack, for example.

"The dress rehearsal has taken place, and it wouldn't surprise me to see these situations happening more frequently," he said.