The short answer
Ice dams form when heat escaping from the house melts snow on the upper roof, and the water refreezes at the colder roof edge and gutters. The ridge of ice traps more meltwater behind it, which can back up under shingles and leak inside. The lasting fix is keeping house heat out of the attic with air sealing and insulation, plus clear, well-pitched gutters and proper roof-edge underlayment.
Why they form
University of Minnesota Extension explains that ice dams form when part of the roof is above freezing while the edge is below it. The main heat source is usually warm air leaking from the house into the attic, along with heat from ducts, chimneys and poorly insulated ceilings.
St. Louis winters swing above and below freezing often, and freeze-thaw cycles after a snowfall are exactly when ice dams build.
How to prevent them
First, seal air leaks between the house and the attic. Then add insulation, and make sure the attic is ventilated. A weatherization contractor can find heat loss with a blower-door test and infrared camera.
Keep gutters clean and pitched toward the downspouts so meltwater can drain. When a roof is replaced, ice-and-water underlayment along the eaves adds an extra layer of protection.
What not to do
Don't climb on an icy roof or chip at ice with tools; it is dangerous and damages shingles and gutters. Channeling the dam is only a temporary emergency fix.
Do gutters cause ice dams?
No, the cause is heat loss from the house. Clogged or sagging gutters can make things worse by holding water and ice at the roof edge, so clean, well-hung gutters help.
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Related: Roof Repair · Gutter Guards