The ever-shifting floodplain

On a National Wetlands Inventory map, a floodplain looks monolithic: a big emerald-green blob with a label like “PFO1A,” indicating a Palustrine,*  FOrested, broad-leaved deciduous (i.e., hardwood) wetland which is temporarily flooded. And the common term “floodplain” suggests a low-lying, flat area. But as I walked around the DeKalb Wildlife Management Area in Decatur County, I quickly understood that the site is in fact structurally-complex, and dynamic. These first three photos (above) were taken within a few steps of each other, one being at an elevation intermediate to the others, those being perhaps my standing height above or below the middle site. A glance shows you just how variable the floodplain can be.

The flat areas at the different elevations (so-called terraces) also have meandering, intermittent drainages (swales) which show signs of regular, substantial water flow. Such swales are significant forces, carving channels and washing away the sandy soil from beneath the trees. One unfortunate toppled giant, an Eastern Cottonwood (Populus deltoides) had little soil around the exposed roots, causing it to lean towards the stream channel over time before ultimately toppling over.

DSC_0753From the tree’s perspective, this was a slow-motion catastrophe. It probably took years of intermittent flow in that swale to undermine the base of the tree, all the while exposing the roots to attack. I noticed the surfaces encrusted with moss growing on the surfaces of the exposed roots, and some large shelf (bracket) fungi attached there as well.

DSC_0754Nearby, I stood atop a sheer bluff, taller than my house, and saw where Long Creek made a bend and carved into the bank. The woods are crisscrossed by swales and other shallow drainages, and I walked amongst the drift lines of logs and limbs and leaves and silt left behind as the swollen streams receded. I saw weedy trees such as Boxelder (Acer negundo), well-adapted to an ever-shifting environment, with a toppled one having suffered a similar fate as that big Cottonwood. I’m confident I could revisit these woods again and again, and never see the same place twice.

But if you stop by, be sure to leave a comment or two about what you find. Enjoy your wetland walk!

 

*Palustrine implies a freshwater wetland in the interior of the continent, lacking significant flow-through. Please use that valuable information wisely. It can include a variety of common wetland types such as sloughs, potholes, marshes, and bottomland forests.

Author: Paul Weihe

Associate Professor of Biology at Central College, traditional author (Textbook of Limnology, Cole & Weihe, 5th ed.; Waveland Press), and now...blogger!

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