With a pretty deep snowpack in these early days of spring, a lot of folks are wondering about the potential for flooding. Well, first of all, we need to talk about the two basic types of floods at this time of the year. There is ice jam flooding, as residents of Fort Fairfield and Perth-Andover know all too well. Then there is general river flooding, or, as I like to call it, all-river flooding. That term means that all communities along the river will flood, rather than only those near an ice jam.
Let’s cover ice jams first. As warmer temperatures arrive, snow begins to melt, the meltwater flows into the rivers and the ice can be lifted from below and start to move. But because the ice is in pieces, some of them very large, they can move in a way that blocks the normal river channel, causing the water to seek a different route. Also, when ice jams release, they can cause very rapid rises in water levels downstream.
It should be pointed out that ice jams are not limited to larger rivers. There is a video I show in my presentations where a tiny mountain stream, along which people are hiking, very suddenly becomes a roaring torrent, with the only clue having been the loud noise approaching. These people knew what the sound was, and climbed to safety just in time!
In terms of “all-river” floods, such as the one which hit the St. John Valley in May of 2008, they don’t happen unless you get an added ingredient to the snow melt, and that added ingredient is heavy rain. Even though the winter of 2007-08 was the record winter for snowfall in northern Maine, flooding of the magnitude seen would not have occurred had not a heavy rainstorm dumped over 3 inches of rain over a fairly wide area.
So major river flooding, which affects all communities along a river, does not happen from snowmelt alone. But if you have a big rainstorm, over a large area, that is concurrent with snowmelt, look out!
Ted Shapiro holds the Broadcast Seal of Approval from both the American Meteorological Society and the National Weather Association. An Alexandria, Va. native, he has been Chief Meteorologist at WAGM-TV since 2006. Email him at tshapiro@wagmtv.com.
