When it is -30 degrees how do cities supply water?

the vortex only lasted a few days. it takes a long season of extreme cold to freeze the ground to any significant depth. most water pipes are buried at least 5 feet, and in cold country, 6 feet so they never get below freezing. the water in an above ground reservoir might form a thin ice layer eventually, but it would take a month or more to happen, that is because of both the heat capacity of the water making it slow to cool and the bottom being in contact with an above freezing ground surface. even northern lakes rarely freeze beyond 3 foot depth over the course of an entire winter.

bottom line, you worry too much. where i lived in Wyoming we had one exceptionally cold winter, and the city water pipe to my house ran under the street. well a few years before, they lowered the grade of the street, so that the buried pipes were only 3 foot or so depth, so they had us run a dripwise stream of water continuously (we weren't charged), which kept bringing warmer water into the shallow pipe area and that prevented a freeze up. keeping water running is a temporary solution when freeze is possible.

Water intakes from lakes and reservoirs are located well below the surface, where it doesn't freeze. And water pipes are buried below the "frost line", which is the depth of soil that can freeze.