The South Saskatchewan River peaked at midnight at a speed of 5,460 cubic metres per second, according to the city's Facebook page.
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The waters are now starting to recede, said Medicine Hat Mayor Norm Boucher, adding the process will likely take days.
However, "that could change," said Ron Robinson, the director of emergency services. Currently, the waters are receding at a speed of 40 cubic metres per second, he said, but that could change to as slow as 20 cubic metres per second.
Despite the lower speed, the river is flowing through the city higher and faster than ever recorded, CBC's Bonnie Allen reported Monday morning, standing near the river, which had risen about a third of a metre overnight.
These "historic amounts of water" are putting pressure on the precautionary measures installed over the weekend, said Allen, including containing walls and sandbags.
Emergency officials remain on high alert as the waters could spill over these walls. Already, city hall and the city's arena and baseball diamond have experienced some flooding.
In the area near the arena and baseball diamond, the precautionary measures are holding up well. Edmonton police officers built a sandbag wall on a residential street that, so far, has managed to keep the water at bay.
Residents must stay away from the evacuation zone. Residents who do not comply with this order may be fined up to $10,000 and face a possible sentence of up to one year in jail, according to the city's Facebook page.
A re-entry plan is under development, said Robinson.
About 10,000 residents remained out of their homes following mandatory evacuation orders.
The city opened two emergency care shelters to house people displaced by the floods.
"Streets without water damage west of Maple Ave will open immediately. "This applies to Maple Ave west from Allowance Ave overpass to First Street."
Areas under mandatory evacuation east of Maple Avenue remain closed.
A large percentage of homes and businesses in that area has been impacted by the flooding, said Robinson, including some banks that had trouble sealing their vaults.
Two bridges connecting the city's north and south sides, which are separated by the river, re-opened on Monday.
The Trans-Canada Highway is open, despite concerns late Sunday that it may have to be closed. Closing the highway would have cut off the only remaining passage between the city's two sides.
The majority of schools are closed today, and municipal employees have been asked to stay at home.
Emergency workers and volunteers spent the weekend preparing for Monday's expected river peak.
"Those extra couple days can make a big difference," said Allen, adding it gave the city more time to prepare sandbags, recruit volunteers and enact their emergency plan.
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"[Environment Canada] gave us a high number, which was actually a good thing," said Boucher. "Sometimes when you scare us, then we can prepare at the higher level."
Earlier, officials had anticipated the river would peak Monday morning at a speed of almost 6,000 cubic metres per second.
The big influx of water was supposed to rush into the part of the South Saskatchewan River flowing through Medicine Hat from two rivers at the same time. Now, it appears that the water will not come simultaneously but in stages throughout Monday and into Tuesday.
The city dispatched emergency workers to both sides of the city, anticipating a closure of the Trans-Canada Highway.
Workers and volunteers piled sandbags around city infrastructure, including the power plant, water plant and city hall, which is now experiencing some flooding.
"With support from Canadian Armed Forces, volunteers and city crews, we are as ready as we can be," read the city's Facebook page.
In Medicine Hat, there is a 60 per cent chance of rain and thunderstorm this afternoon, according to Environment Canada.
Video of the flooding from the City of Medicine Hat.
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