How a dam forms in this climate specifically
Heat escaping from the house into an under-insulated or poorly ventilated attic melts the underside of the snow sitting on the upper roof. That meltwater runs down under the snow layer until it reaches the eave, which overhangs past the heated wall and sits at outdoor temperature, and it refreezes there. Over a few days of that cycle a ridge of ice builds at the eave, and meltwater pools behind it and eventually rises high enough to get under the shingles and into the house.
Our winters make this worse than a milder climate would because the freeze and thaw swings are sharp rather than gradual. A week of hard cold followed by a sudden warm afternoon moves a large volume of water fast, and an attic that handled a normal winter fine can build a serious dam in that one stretch.
Steam, not hammers or salt
When a dam has already formed and water is coming into the house, the safe removal method is low pressure steam, which melts a channel through the ice without gouging the shingles underneath it. Chipping at a dam with a hammer or an ice pick breaks shingles and can put a hole straight through the roof deck. Rock salt or calcium chloride spread across a roof corrodes metal flashing and gutters and runs off into the yard and any nearby plantings.
Steam is slower than a hammer but it is the only method that does not create a second repair on top of the leak you already have.
What actually stops it from coming back
Removing the ice that is already there does nothing about why it formed. The real fix is stopping heat from reaching the attic in the first place: sealing air leaks around the chimney chase, plumbing penetrations, recessed lights, and the attic hatch, then bringing insulation up to a proper depth across the whole attic floor. After that, ventilation matters just as much as insulation, soffit intake and ridge exhaust working together to keep the whole attic cold enough that the snow on the roof does not melt unevenly.
We check for the small things that undo an otherwise good attic too, like a bathroom fan that vents into the attic instead of through the roof, which pumps warm moist air into exactly the space you are trying to keep cold.
Ice and water barrier as the backstop
Even a well-sealed, well-ventilated attic can see some ice in a genuinely hard winter, and the self-adhered membrane installed under the shingles at the eave during a roof replacement is what keeps that ice from becoming a leak. We run that barrier further up the roof than the minimum code requirement, and across every valley and wall intersection, because those are the spots where drifting snow sits the longest.