When the temperatures turn mild, the snow starts to melt, and spring is finally on the doorstep! However, when the temperatures go up too quickly, all that winter snowpack tries to melt all at once. That's not good for anyone.
There's a lot of water tied up in a snowpack. Even where the snowfall's usually powder snow, the snow packs together over time, which makes the water content much higher. By the time the snow starts to melt in earnest, more than half of that snowpack could be water content. When the temperatures go up too fast and the snowpack melts rapidly, imagine that much rainfall falling all at once!
It gets worse. At least some summer rain soaks into the ground, but when a snowpack melts that fast, the ground's still frozen until late into the melt. That means nearly all the meltwater goes straight into the rivers. Any extra precipitation joins it.
That's just about the most dangerous flood conditions possible. First, spring river crests are the highest of the year, and they hit record heights after a rapid melt. Second, the frozen ground on the riverbank is highly vulnerable to erosion, so riverbanks are undermined and all kinds of broken branches and other debris are swept into the river. Third, rivers in spring flood are fast! With all that debris, erosion happens even faster. And fourth, the water's still less than a degree away from freezing cold, and will stay that way until nearly all the snow along the watershed has melted.
Those kinds of floods aren't limited to the places with snow. Rivers feed into larger rivers. All that floodwater concentrates on one big river. If it flows far enough south, like the Mississippi River, then that huge flood crest is carried downstream into places that never see snow!
Blowing up the ice jam on the Rideau River is an annual rite of spring in Ottawa. The elevation change in the river's not large enough to stop large sections of ice from building up between Billings Bridge and Rideau Falls. Without the dynamiting, the water would back up behind the ice jam and flood parts of Ottawa. That's in a normal year.
When there's a large, rapidly melting snowpack and the upstream part of the river thaws more quickly than the downstream part, ice jams will force record amounts of floodwater back on itself. Without dedicated floodplains or a specially built spillway, there's nowhere for all that extra water to go. That's the kind of thing that turns into floods of epic proportions.
In the fall and winter, rain and snow gets into cracks in the rock and freezes there. That pushes apart the rocks, but the ice can keep them in place, until it melts in spring. A rapid melt means that all that ice gives way at the same time. On a cliff or mountain slope, the result can be serious landslides.
When the ground thaws, seeds sprout and other plants are ready to wake up. Under normal conditions, that's perfect timing to take advantage of the water released when the winter snow cover melts in spring. This is what replenishes soil, wetlands, and ground water every year.
However, when the snowpack melts too rapidly, the ground's still frozen. That means that the plants are still in winter mode, so they can't take advantage of the meltwater. And none of that water can make it into the frozen soil for later. It just runs right off! All that fresh water goes downriver and is lost.
The only way around this is to let a river keep broad floodplains without building on them or dyking them. Then the river spreads out and slows down, so that instead of carving out the banks and eroding the topsoil, the floodwaters bring rich silt and spread it out evenly. After the river crests, the floodwaters go down gradually in the floodplain.