You know you’re not from a “normal” part of the world when you glance over the side of the bridge and exclaim, “Wow! There’s water in the river.” And boy was there ever! A week ago Saturday, a line of thunderstorms dropped lots and lots of hail and rain on the headwaters of several branches of the Red River. When I crossed the river down near Esteline around noon on the next day, so much water had flowed downstream that the river filled the banks and had large waves on it. Those people passing through from out of state to the east probably wondered if someone was reenacting the Plagues of Egypt. The river really did run red, as if with blood. (Thus the name.) High, wide, muddy, and dangerous, the water raced downhill, headed for the Gulf of Mexico.
Two days later, when I crossed it again, more sand and mud than water met my eyes. The Red is a wide, braided stream because of all the sediment it carries (and the relatively flat land it flows over). The main channel had some water in it as it snaked and swept back and forth across the valley floor. But far more bare mud and sand met my eye. Now, I wouldn’t go walking through the valley, because of quicksand, but I nodded to myself. It looked like a normal local river. Very flashy.
The hydrograph of a flash flood looks like this:
This is from a pretty good paper at ResearchGate.net, “Flash Flood Hazards” a chapter from a water management book, by Denes Loczy, Szabolcs Czigany, and Ervan Pirkhoffer The X-axis is time duration of event, and the Y-axis is flow volume. You notice that instead of a nice slow rise, smooth peak, and gradual fall (a stream with snow-melt as a source, for example), you see a vertical increase, rough curve, and then only slightly slower flow drop. That’s a flashy stream. In the US, most Great Plains, High Plains, and western streams are flashy, prone to flash floods. Fast come, fast go.
Flashy streams are useless for, oh, hydropower, irrigation, running mills, sailing boats up, you know, economically useful things. They come, they go, they go away. Not helpful, unless you can trap that water. In some cases, they are lethal, as the poor folks discovered last year when the far upstream rainstorm caught people in a slot canyon in Utah. In the case of the Red, Canadian, Picketwire, Platte, Green, Colorado (both of them) and others, so much sediment travels with the river that it will fill dams, making the dams into lovely, tall waterfalls eventually. That’s not useful for irrigation or supplying urban drinking water.
If you look at a small stream under a large bridge and wonder if the state/county construction people lost their minds, you might be close. They probably pulled a lot of hair out, trying to design a bridge that would stand up to that two-times-a-year flood event, but not cost so much that the state/county screamed. “Why’d they put such a big bridge over that?” is an oft-heard complaint. Come back four or six or fewer hours after a big ‘ol thunderstorm, or a enormous wet low-pressure system dumps rain for a week straight, and you’ll understand why.
I love logging onto the USGS stream gauges for the Canadian and other local rivers and watching floods in real time. the curve goes “flat-flat-bump-liftoff!-crash!”
*shrug* What can I say? I’m a hydrology nerd. I like reading rivers.