Lethbridge declared a state of emergency as levels in its reservoir dipped critically low. Alberta Health Services also put a boil water order in place saying water out of the tap is no longer safe to drink without bringing it to a rolling boil for at least one minute.
People were asked to shorten showers, avoid using dishwashers and refrain from doing laundry. Flushing toilets was still OK.
Pools and arenas were closed and car washes were asked to stop operating. A ban was placed on open fires to avoid a potentially dangerous blaze that would be difficult to fight without enough water.
Officials said the quick snow melt due to suddenly rising temperatures was causing high turbidity in the Oldman River from which the city pulls its supply.
That murkiness was making the water extremely difficult to treat. Production at the water treatment plant was shut down, so the city was unable to restore storage levels.
"At this point we're not refilling the reservoirs," said Brian Cornforth, the city's emergency service director. "We did not put on a local state of emergency lightly. This is a critical step for us.
"We need you to cease using water for any purpose other than eating and drinking."
As of about noon on Wednesday, there was enough potable water left in the reservoir for about eight hours under normal circumstances.
Cornforth asked Lethbridge's 90,000 citizens not to hoard tap water either.
"That will put pressure on our system and we will have to take drastic measures in the event of running out of water."
Doug Hawkins, Lethbridge's director of infrastructure, explained why it was proving so difficult for the treatment plant to process water from the river, which is still covered in ice.
"The runoff is occurring actually undiluted. It's running across the top of the ice and directly into the intake at the water treatment plant," Hawkins said. "The organic material, dissolved material, that's in the raw water right now is unprecedented in terms of what we've seen in the past and had to deal with in the past."
He said water conditions haven't been this bad since severe flooding in the region in 1995.
"We've been in situations where we've had trouble treating the water and had to shut down in the past ... but never was it as troublesome to treat as it is this year."
The city says the heavy runoff is expected to continue.
Smaller surrounding communities were also being asked to conserve.
— With files from Pat Siedlecki and Tina Giesbrecht at CJOC
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