May 2011 will not be forgotten by anyone connected with the Mississippi River. While a few historians claim the lower Mississippi River has been higher, the 2011 flood set a new high-water record on the Vicksburg gauge, and was just inches shy of record highs on the Greenville and Memphis gauges. The social and economic damage were huge.
I’m a fisheries biologist, so I’ll leave resolution of these problems to others. What was the effect of the flood on the fisheries?
I’ve spent quite a bit of time on the Mississippi River since June tracking pallid sturgeon to better understand this endangered fish’s habitat requirements. You can take in a lot about a river when you are idling downstream at 6 mph listening for the “chirp” from a pallid sturgeon bearing a sound-emitting transmitter.
I was surprised at how many camps appeared undamaged, but I have friends who live along the river, so I know better. The greater-than-usual amount of activity around the cabins was more likely people repairing their flood-soaked cabins than enjoying them. Many ramps were sanded in and unusable, and undoubtedly many roads from the levee to the ramps remain impassable. Yes, access to the fisheries is impaired. Hopefully it will be restored by the time you read this. Sadly, the flood-caused financial woes to some privately maintained boat ramps and stores were overwhelming and signaled the end of an era.
While the people-side of the fishery was hard hit, the fish will benefit in the forthcoming years. As I floated the river like a high-tech Huck Finn listening for sturgeon, it struck me how Old Man River looked as he has looked every summer for the past 30-some years that I have been doing fisheries research there. The habitat hasn’t changed.
The floodplain connection
The floodplain is essential to the fish. The accepted model for fish production in floodplain rivers, like the Mississippi, is that the plants and animals produced on the floodplain fuel, via the food web, the fish. The model is called the flood pulse concept. This is in sharp contrast to lakes, where phytoplankton — the algae suspended in the water — convert solar energy into plant material that then spirals through the food web.
Said another way, fish production in a lake is determined by organic matter produced in the lake; fish production in rivers is determined by organic matter produced on the watershed.
The spring flood, if high enough, provides fish access to the abundant food resources on the floodplain. The inundated floodplain also provides essential habitat for spawning and the early life stages of fish that spawn in standing water — fish such as shad, sunfish, crappie, white bass, blue and channel catfish and buffalofish. Indeed, more than one third of the 140 species of fish that live in the Mississippi River depend on the floodplain to complete their life cycle.
When the flood waters recede, the plant and animal material, as well as the young fish spawned on the floodplain, are pulled into the river. What reaches the river then provides food for riverine fish — those that remain in the river.
Without floods, the fish are denied access to the abundant production on the floodplain, and fewer young fish are produced to keep populations abundant and to feed the riverine fish.
Well, that’s what the flood pulse model predicts, but this model was developed primarily from studies in tropical rivers in Africa and South America. Does the model apply to the Mississippi? Studies on the impounded upper Mississippi River found, as predicted by the flood pulse concept, increased growth of floodplain-dependent fish during the flood of 1993 when the Mississippi River floodplain was inundated from late spring through summer.
Conversely, two assessments of fish production in the lower Mississippi River failed to demonstrate a link to floodplain flooding. The lack of a floodplain effect was attributed to the fact that the levees have separated the Mississippi River from about 90 percent of the historic floodplain, and there wasn’t enough floodplain to benefit the fish.
The lack of a functional floodplain in the lower Mississippi River is a misconception. Yes, the levees have separated the river from much of its historic floodplain, but 1.6 million acres of batture land (the floodplain between the levees) still remain.
Studies by my fisheries graduate students at Mississippi State University initially failed to find a relationship between floodplain flooding and catfish growth. However, when we looked at the data more closely, we found growth of blue and channel catfish was, as predicted by the flood pulse concept, greater in years when the floodplain was inundated for at least several months and the water temperature was above 65 degrees.
This makes sense. Catfish, like the other fish in the lower Mississippi River, are warm-water fish and don’t feed or grow much in cold water. Fish farmers feed channel catfish very little until the pond temperatures climb into the 60s. So, the floodplain does contribute to blue and channel catfish growth when water temperature is considered. The flood pulse concept applies, at least to blue and channel catfish, in the lower Mississippi River.
We did not find flathead catfish growth related to the length of time when the floodplain was flooded, even when water temperature was considered. This, too, makes sense — flathead catfish don’t move onto the floodplain.
Back to the flood of 2011. Bad for people who fail to understand that the Mississippi River is a dynamic and powerful system, and the floodplain is as much a part of the river as is the main channel. Good for fish that were able to spawn and feed on the flooded land when the water was warm.
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