When do you expect flooding?

It depends very much on where you are, of course, and what causes the flooding. Here in interior Alaska we have two flood seasons, with two quite different mechanisms, and we’re starting into one of them now, at the driest time of the year.

Yes, April is our driest month and May, while a little wetter, is the second driest month with temperatures above freezing. So why flooding now? Why not in the rainiest month, August, which did produce the great Fairbanks Flood of 1967?

Two words: ice jams.

We’re already getting some information on river breakup and flood warnings. In fact, there is an ice jam flood advisory on the Tanana River near the mouth of the Salcha (upstream of Fairbanks) today and tomorrow. Most of the rivers in Alaska run from highlands with snow — lots of snow — in the winter months to coasts that during the spring are a lot colder than the interior where the snow is melting under the sun. The snow melts and the meltwater runs into the river while the lower reaches of the river are still frozen. As the river rises and the sun weakens the ice that has covered it over the winter, the ice breaks into chunks and slabs and begins to flow with the river. It used to be axiomatic that rivers could not be crossed, and bridges were often swept away, during breakup, when the ice is carried downriver. When I came to Fairbanks it was still a possibility that the last car of the year over the ice bridge across the Chena River would go through the ice. The great gambling event of the Alaskan Spring is the Nenana Ice Pool – wagering on the exact day, hour, and minute that the ice will go out at Nenana, Alaska.

But all that ice moving downriver can cause problems, too. It’s not wimpy, thin ice; the ice at Nenana is over 2’ thick today, and while it’s thinned from is original 3 ½ feet, it still makes big chunks. If it piles up, as may happen at a different place every year, the flow of water is severely impeded, and the water spreads out over the adjacent land. Rivers in Alaska are transportation corridors – not only for boats in the summer, but for dog teams and snow machines in the winter. Consequently most of the older settlements in Alaska are on this precursor to our skimpy road net, and they almost expect to be flooded in spring. Fuel lines and tanks must be tied down, as must boardwalks, lest they float off. Belongings are put up high. I expect the public service announcements warning residents to prepare for flooding to start soon.

By the way, notice that our wettest month has an average precipitation of under 2″, and the annual total is only 10.31″. In terms of total precipitation, I live in a desert.

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