KALAMAZOO, Mich. — An ice jam caused flooding on the Grand River near Portland, Michigan, on Wednesday, Feb. 6, 2019.
In cold weather, ice forms in large sheets across rivers and streams. Then, when the temperatures warm up, that layer of floating ice begins to break up, and the water currents start pushing the chunks downstream, and into one another.
As the chunks float down the water way, they can accumulate in certain areas and block the water flow. Accumulations tend to occur in spots where the rivers bend, at mouths of tributaries, at points where the slope of a river decreases, and downstream from dams or upstream of bridges.
When the pieces of ice gather in these areas, the water that is held back could cause flooding upstream, because the flow of water is blocked.
Then, if the ice jam suddenly breaks, flash flooding might occur downstream.
The National Weather Service in Grand Rapids said the flooding from the morning of Feb. 6 in Ionia County likely occurred as a result of high flows from the recent rain and snow melt, but also because of an ice jam that has formed downstream of Portland.
Three to five consecutive days with high temperatures in the 40s typically creates enough melting power to break up the ice on rivers and streams.
Air temperature is an important factor in melting snow, even with rain falling on top of the snow. Rain alone usually will not add enough heat to the process.
At 40 degrees, one inch of rain will produce about a tenth of an inch of added water from snow melt. Frozen ground will result in more water running off directly into water ways. If rain does fall, then three to five consecutive days with high temperatures in the middle 30s might result in the ice breaking up.
So ice jams can occur within a day or two after the rain begins.