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You have certainly done business on them, and you may well have lived within their boundaries. Whether you recognize it or not, and whether you are reading this in the desert West or the soggier regions of the country, floodplains are a part of the landscape around you — and they can be highly dangerous places to be.
A floodplain is the flat part of the Earth beside and around a river. It’s also the place we like to build houses and schools and stores because it’s easier to build on flat ground rather than a steep hillside. Especially in the old days, when materials came by boat to a town, or perhaps by railroads (built on level floodplains because train engines don’t pull railroad cars up steep ground), entire towns were built on floodplains, with hills around them sometimes left less populated.
It’s a recipe that works well enough, most days. But when conditions change away from the normal, of course, it’s disaster for whole towns and their inhabitants.
A river can only move so much water downstream at a particular point per minute. If it’s full to the brim and moving as fast as it can, it doesn’t have the capacity for more. The technical term is “bank-full” and the stream cannot accommodate more water than that without flooding.
A flood is a natural event in which the stream simply occupies the floodplain as well as the stream channel. To put it another way, the floodplain is a natural part of the stream system. It’s just a part of the system that a stream or river occupies only occasionally, rather than everyday.
But that’s the part it’s tough for us humans to graciously accept. We see flat land and we tend to start building: hotels, stores, hospitals, libraries and more.
Most of the time, we get by reasonably well with our decisions of where to locate our installations, ranging from concrete monstrosities to basic campgrounds. But from time to time this time of year we see on the news another example of how a stream can flood, including the special and terrifying case called a flash-flood.
In a normal flood, water overtops its banks in a fairly predictable way, one in which the authorities can warn people about. Indeed, the flooding of a major river is a disaster often recorded in slow motion.
Flash floods are very different because they hit so quickly. They often affect smaller streams. If a deluge occurs above part of such a stream, the water level will rise rapidly. In the case of the flash flood in Arkansas that killed campers along the Caddo and Little Missouri rivers, water levels rose as swiftly as 8 feet an hour.
Because the rainstorm – a true Noah-like deluge — occurred in the night, people never saw it coming. Even if they had been awakened with warning bells of some sort, many of them still would have lost their lives because the side slopes of the hills they were in were so steep and some of the roads out of the area were blocked by floodwaters. The violence of the torrent increased as the night went along — with asphalt being torn off roads by the currents at one point.
Flash floods occur in the driest deserts, too. That’s because of two factors. One is that rainfall in deserts tends to be either non-existent or extreme. In other words, there aren’t many gentle rains in the desert. And the ground in the desert is often covered with a crust-like material that slows or even prevents water from percolating into the ground — natural pavement, if you will.
So if you camp in the desert this spring or summer be sure — doubly sure — not to pitch your tent in an arroyo or any other type of “dry stream.” That’s been a fatal mistake made by too many people not thinking of what heavy rainstorms upstream can mean for local conditions.
Dr. E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard. Planet Rock Doc, a collection of Peters’ columns, is available at bookstores or from the publisher at wsupress.wsu.edu or 1-800-354-7360. This column is a service of the College of Agricultural, Human and Natural Resource Sciences at Washington State University.