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Essays on Floodplains–Part 2, the Forests

For the second time in two weeks there is an article about the importance of floodplains on the Upper Mississippi River.

Susan P. Romano discusses the decline of floodplain forest in “Our current understanding of the Upper Mississippi River System floodplain forest”, appearing this month in Hydrobiologia, which is only available through Springerlink at a cost of $34.

From St. Paul south to the mouth of the Iowa River, the navigation channel of the Upper Mississippi weaves through c-shaped islands. From the Iowa south the island are narrower and the main channel of the river tends to run to one side of a collection of islands or the other. This is the floodplain of the Upper Mississippi.

Wigwam Slough near LaCrosse, Wisconsin

The early explorers of the Upper Mississippi found a floodplain forest of oaks, shellbark hickory, northern pecans, and hackberries–hard-mast trees that produce nuts, food for wildlife, incased in a hard shell–mixed with silver maple, cottonwood, willow, and green ash–trees that produce wind-blown seeds. Late twentieth century explorers of the Upper Mississippi found a forest dominated by silver maple and willow, mixed with some cottonwood and green ash.  Most of the oaks and hickories on the Upper Mississippi floodplain were lost to logging in the nineteenth century. Those that survived the loggers were flooded out with the construction of the system of locks and dams that produced the nine-foot navigation channel in the twentieth.

Silver Maple edge Heiser Slough, near Muscatine, Iowa, June 1995

Their location in the floodplain determined which trees survived the steady water levels required by the nine-foot navigation channel. Willow, silver maple, cottonwood, and green ash: all tolerate extensive flooding and grow on the lowest and wettest soils in the floodplain. Oak, hickory, pecan, and hackberry tolerate a little flooding, but not the permanent flooding brought on by the nine-foot channel. Those that survived the completion of the dams grew on drier soils on ridges that rose a foot or two above the floodplain.

At the beginnng of the 21st century silver maple dominated the floodplain, but the maples are growing old. While they produce millions of winged seeds every spring, which sprout and grow before the parent trees leaf out, they do not thrive in their parents’ shade. The maples are not replacing themselves.

Maple seedlings along Skunk Slough, Upper Twin Island, near Burlington, Iowa, June 1994

The Flood of 1993 devastated the floodplain forest from the mouth of the Iowa River south, with the loss of trees escalating as the river flowed south.

Forest along Lake Odessa bordering Pool 17 near Muscatine, Iowa, June 1995

Forest managers, who surveyed the Upper Mississippi floodplain after the flood, found that the flood had drowned almost all the hackberries, which grew on the ridges south of Rock Island. In the wake of the flood forest managers with the Corps of Engineers and the Fish and Wildlife Service planted hard-mast trees where hackberries had grown before the flood. They found they could walk away from their plantings with reasonable assurance that the young trees–at least 5/8 inch in diameter and four feet tall–could survive all but the most extensive floods. And, they could trust that the winged seeds of the silver maple, cottonwood, ash, and sycamore would take root and create a diverse forest.


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