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Environment
South Slave

Why was this year’s Hay River flood so bad?

Last modified: May 13, 2022 at 11:59am

We weren’t supposed to have to evacuate the whole of Hay River any more.

The whole point of the “new town” – founded almost 60 years ago – was to end the danger that resulted from living on Vale Island, in prime flooding territory between two river channels.

But this year, kilometres away from Vale Island and on higher ground, downtown Hay River flooded anyway.

What happened?

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A large part of the answer is an unfortunate coincidence: a storm system parked over the entire basin of the Hay River just as breakup reached a critical point.

Breakup is always a lottery. Ice jams play a big role in flooding and they are difficult to predict or control. Some years, the jams are less dangerous. In others, they are lethal.

But this year, inconvenient ice jams at the ends of two river channels were made much, much worse by the storm and the sheer amount of water it sent into the Hay River system.

When that rain and snow fell, it hit a landscape that was already saturated.

Well before the flooding began, the NWT government had warned that in parts of the Hay River basin – the massive area in which all water drains toward the Hay River – water levels were “at or near the highest levels ever recorded” for this time of year.

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A GNWT map of snow water equivalent in April 2022. Blue is wetter, red is drier. Much of the Hay River basin, in the southern NWT and northern Alberta, is blue.

The ground being heavily saturated “means most water will flow directly into rivers, creeks, lakes, and streams – which will likely increase the already high water levels,” the territory stated in late April.

That is exactly what came to pass.

As we progressed into May, water levels on the Hay River began to push toward the very top of the charts.

This GNWT-supplied graph shows the water level near the Hay River’s mouth reaching a height beyond any other recent year in early May, just as flooding began. Compare that with charts on the same day for Great Slave Lake in Yellowknife or the Mackenzie River in Fort Simpson.

Last weekend, a storm system arrived that didn’t simply dump rain and snow on Hay River. It did the same all the way up the basin, over all the tributaries that feed the river.

That storm could not have happened at a worse time.

“With Hay River, it’s the timing of this quite extraordinary precipitation event coinciding with breakup,” NWT hydrologist Shawne Kokelj told Cabin Radio on Tuesday.

“What is unusual this year is the timing of this large storm event. That’s what is exacerbating this already challenging time of year.”

The Hay River basin, shown in blue, spreads across much of northern Alberta and parts of BC.

The image above shows the extent of the Hay River basin, highlighted in blue.

In the video that follows, you can watch a representation of that storm system. Most of the centre of the screen is occupied by the Hay River basin.

This sequence uses data from GFS – one of the world’s leading weather models – to simulate the evolution of the storm over the Hay River basin. The map shows forecast three-hour accumulated precipitation, updated hourly, from Friday to Monday on the weekend of the storm.

The storm came after a period of warm temperatures, then brought a dramatic chill.

Kokelj says that sudden downswing in temperature could have contributed to the ice remaining jammed at the end of the channels for so long.

If that ice had shifted, the amount of water entering the system would have been far less concerning as its path would have been much clearer to empty into Great Slave Lake.

Instead, huge amounts of water – with nowhere else to go but into rivers, given the land was so saturated – poured toward Hay River, where a plug had been placed at the river mouth.

And the storm kept going.

“It parked over the entire basin,” said Kokelj on Tuesday. “There’s still high water coming from farther upstream because a lot of rain fell there, too, and now some of the snow is melting and so keeps feeding a lot of these smaller streams.

“As soon as you stop a river, the volume of water seems incredibly large and it backs up quickly.”

This year, there was so much water – with so long to back up – that even the new town eventually came under threat.

In the simple model below (this isn’t based on real flood data, it’s a simulation of how the town would flood based on land shape alone), it takes a long time for the new town at the bottom of the image to flood. But in 2022, the conditions were right.

This rudimentary animation by Everett Snieder shows how water would be expected to inundate Hay River based on the elevation and shape of the land.

Given the storm happened several days before the flood, and we knew a huge amount of water was falling over already-saturated land and proceeding to a river susceptible to ice jams, was there any more that could have been done? That will be scrutinized in the weeks to come. (The GNWT is still reviewing last year’s floods – the process takes months.)

However, there are two factors that help to explain why this week’s flood still wasn’t readily predictable.

The first is our poor understanding of ice jams.

Hay River is a mecca for ice jam research. There have been at least seven different ice jam studies carried out on the river in the past 30 years, according to a specialist water security unit at the University of Saskatchewan.

But despite all that work, we still don’t really know exactly when and how an ice jam is going to form – and precision is key, because the exact location and timing of an ice jam determines how much water will back up behind it and where that water will go.

The same university states: “An efficient and effective predictive tool is still a work in progress.”

Kátł’odeeche First Nation as seen on the morning of May 12, 2022. Image: Mikey McBryan

The second factor is the quality of the data.

Ice being shoved down a river often ends badly for the nation’s network of hydrometric gauges.

“We generally anticipate the gauges are going to get taken out by ice, so that makes it challenging,” said Kokelj.

That means the readings hydrologists need at critical periods are often somewhat unreliable or may disappear entirely, complicating the task of understanding what is happening at different locations.

Speaking of different locations, Fort Simpson spent the week declaring its own flood risk had ended even while Hay River sank further from view.

Kokelj stressed the behaviour of the Hay River usually has very little bearing on what happens along the Mackenzie in places like Fort Simpson or Jean Marie River. A bad season in Hay River does not necessarily mean the same thing will happen elsewhere.

“The percentage of Mackenzie River water coming from the Hay River is very small,” she said, adding that Hay River “is essentially in a delta, so it is quite different from the communities along the Mackenzie River.”

“If it’s flooding in Hay River, that does not mean it’s going to flood in Simpson,” Kokelj concluded.

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