Trouble Ahead for Missouri Valley Forests


It’s been three years since the Great Missouri River Flood of 2011.  Although that may seem like a long time, the physical effects of the flood are still readily visible across the Lower Missouri Valley (the valley south of Sioux City) – in the large sand dunes next to the river, in the scoured banks downstream from the Army’s wing dams, and in the cottonwood forests that fringe the navigation channel.

At the height of the flood, when floodwaters covered segments of the valley south of Sioux City under ten feet of water, biologists worried about the consequences of prolonged submersion on the valley’s extensive cottonwood timber tracts.  Scientists feared that upwards of a million cottonwood trees might die along the Lower Missouri because of the flood’s long duration.  The valley’s cottonwoods hadn’t evolved to cope with months of high water soaking their root systems, bark, and trunks.  Rather, the trees evolved with the river’s pre-dam flow regime, which included brief floods often followed by low flows.  For instance, before the big Dakota dams went online in the 1950s and 1960s, the lower river experienced a short, sharp flood in April.  Then, during the month of May, the river dropped down again, sometimes to levels as low as those recorded in the dry months of August and September.  In 2011, the river stayed high for month after month in a man-made flood never before experienced in the river’s long history.

Unfortunately, the predictions of the biologists are coming true.  It took three years for the flood to kill the cottonwoods, but the trees are now dying in droves.  Between Sioux City and Omaha, thousands upon thousands of trees are dead or dying.  The dead cottonwoods are discernible by their gray barren branches and white barkless trunks.  The towering dead trees are plain to see against the backdrop of green, lush vegetation that remains alive.  Sick, flood-stricken trees are visible by the absence of a full complement of dark, green leaves.  Instead, their leaves grow in odd, misshapen clumps.  Branches that should have a full array of leaves possess long bald sections in between sparse leafy patches.

The big question is what will happen to the cottonwoods that remain alive?  Will they too succumb in the years ahead?  If the Lower Missouri Valley loses its old cottonwood timber stands, the entire river valley ecosystem (what’s left of it) will suffer.  Species that relied on those cottonwoods, including songbirds, turkeys, and deer, will likely experience population declines.  Fish populations may also be negatively affected by the elimination of streamside forests.  Live trees that tumble into the river feed zooplankton and zooplankton feed fish.  Without zooplankton in the river, the Missouri’s fish will go hungry.  Ultimately, what all of this means is that we will be recording the environmental consequences of the man-made flood of 2011 for years to come.

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