As I write this column, the Mississippi River — the third largest river in the world — is 10 feet above flood stage, 2 feet below the record high and almost 20 feet above the average stage for this time of year. It’s a tense time for people who live and work along the Big River.
But it’s probably a good year for fish.
The floodplain — some say the batture — is the portion of the landscape between the river bank and the levees that is routinely flooded during seasonal rises in water level. The present Mississippi River floodplain, even though confined by levees, ranges from several hundred yards to many miles wide. The floodplain of the lower Mississippi River historically extended as far east as Greenwood.
While flooding causes problems for those who encroach on the river’s domain, the floodplain is essential to fish. Oxbow lakes and abandoned channels provide the pond-like conditions preferred by adult crappies, sunfishes and largemouth bass.
During springtime flood pulses, the shallow water and refuge from the current on the inundated floodplain provides essential spawning habitat for a long list of fishes including shads, minnows, buffalofishes and most of our favorite sportfish. After the eggs hatch, the floodplain also provides the slack-water, structure-filled habitat needed by the young, weakly swimming fish.
Of the 109 species of freshwater fish in the lower Mississippi, more than half depend on the backwater habitats provided by the floodplain.
The floodplain also provides a rich food supply for not only the young fish but for all fish in the river. In lakes and ponds, phytoplankton — microscopic algae suspended in the water — and aquatic plants convert the sun’s energy to chemical energy that fuels the aquatic food web.
In rivers, the trees and other terrestrial plant life that flourish in the moist and nutrient-rich soils of the floodplain replace phytoplankton and aquatic plants. The decomposing leaves and plant material is the energy that powers the riverine food web. Rising water connects the river’s fish with this plentiful food supply. Falling water draws these energy sources and the abundant aquatic animals and their offspring into the river, where they provide food to fishes like flathead catfish and sturgeon that shun the floodplain for the river.
Engineering activities conducted in the name of social and economic benefit have altered the once flood-prone Mississippi. The levees have permanently disconnected the Mississippi from 90 percent of its historic floodplain. That is a tremendous loss of aquatic productivity, yet 1.5 million acres of seasonally inundated floodplain still remain.
Less visible but more damaging from a fisheries perspective were 16 cutoffs — man-made channels that bypassed meander loops — completed during 1929-1942. The cutoffs shortened the river by 150 miles, making commercial navigation easier and speeding the downstream passage of flood waters.
The cutoffs worked. Now, in an average year, the Mississippi rises enough to barely spill onto its floodplain, and the flood pulse lasts from only mid-March to mid-May. Before the cutoffs shortened the river, higher river stages pushed water onto a broader floodplain, and portions of the floodplain were inundated from February to July in an average year.
Prior to the cutoffs, the fish benefited from the productivity of the floodplain for at least five months. And the prolonged inundation resulted in warmer water that allowed the fishes of the lower Mississippi to spawn and successfully rear young. The abbreviated and cold flood pulse that now remains is scarcely long enough for fish to benefit from the floodplain’s rich food supply, spawn and rear their young.
If high waters persist into June, 2008 will be a good year for lower Mississippi River fish. Significant increases in growth of floodplain fishes were recorded in the upper Mississippi River in 1993 when flooding persisted into the summer. Research by my students in the lower Mississippi River has shown increased growth of blue catfish during a high-water and longer-flood year.
Restoring the ecological function of the lower Mississippi River is impeded by polarized views that insist that the levees are the problem. Yes, the levees that disconnect the Mississippi from most of its floodplain adversely affect the fish and wildlife in the Mississippi Valley. But the effect of the cutoffs is more severe and also can be mitigated by manipulating the existing floodplain to create longer and warmer flood pulses.
The lower Mississippi River, despite alteration, provides abundant fishery resources and is one of a precious few underexploited fisheries remaining in the United States. Substantial efforts are under way by the lower Mississippi River states to improve people’s awareness of the fishery value of the Big River and access to its bounty.
If the fishery resources are plentiful, is management to enhance the ecological function of the floodplain necessary? I guess that depends on your point of view, but reliable reports of Mississippi River blue catfish over 100 pounds in St. Louis fish markets in the late 1800s make me wonder just how good fishing could be in Ol’ Man River.
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