Chasing the Mud: The Mississippi River Flood of 2011


Narrator (McKee): The 2011 Mississippi River
flood was among the largest and most damaging to occur along this waterway in the past century.
It rivaled other major floods, which occurred in 1927 and again in 1993. To reduce stress on the river levees and control
structures during the spring of 2011, the Morganza and Bonnet Carre Spillways, north
of New Orleans, were opened. These actions caused flooding in some areas, but were done
to prevent catastrophic failure of the levee system and a possible change in the course
of the river. This major flood event dramatically increased
the flow of water and sediment down the main channel of the Mississippi River and its tributary,
the Atchafalaya River. The floodwaters diverted through the spillways could potentially carry
tons of sediment into the marshes and swamps of the delta plain. Scientists scrambled to study this flood event
and to track the movement of sediment carried in the floodwaters. This video follows one
group of scientists as they investigate sediment deposition in the wetlands of the Mississippi
River delta plain. The Mississippi River drains much of the continental
US from the Rockies to the Appalachian Mountains. A vast network of tributaries carries water,
sediment, and nutrients into the main channel of the river where it is then funneled to
the Gulf of Mexico. Every year, the river swells due to spring
rains and snowmelt. Before levees were built, floodwaters escaped the confines of the river
channel and spread out across the floodplain. The sediment and nutrients in the floodwaters
were carried into the marshes and swamps where they nourished the plants and renewed the
land each spring. K. McKee: Water and sediment are the lifeblood
of these wetlands. Flooding is catastrophic for human communities, but it’s essential
to maintain wetlands in a healthy state. Flooding brings in sediment, which drops out of the
water as the water flow slows down, and this accumulates on the soil surface and builds
the elevation of the soil surface upward. Narrator (McKee): In the 1920s, levees were
built along the Mississippi River for flood control, which prevented overbank flooding
but starved the wetlands of sediment. The delta has also been subsiding as the river
sediment settled and compacted and as sea level rose. Over time, marshes gradually drowned
and became open water. From 1932 to 2010, an area the size of the state of Delaware
has disappeared. As sea-level rise continues, more marshes will disappear without inputs
of new sediment to maintain soil elevations. Currently, the river sediment bypasses the
wetlands as it is funneled down the main channel and out of the mouth of the river, which can
be seen in this satellite image as a dense brown plume mixing with the dark blue waters
of the Gulf of Mexico. With the opening of the Morganza spillway, however, more water
and sediment were diverted to the Atchafalaya basin where it fed into the swamps and marshes
and eventually to the gulf. The question scientists wanted to answer was
how much of this 2011 flood sediment was being deposited in the wetlands? To reach the remote study sites located across
the Louisiana coast, scientists use helicopters on fixed floats so that they can land in the
marshes to collect samples. K. McKee: I’m standing in a marsh in the
middle of a delta that’s forming at the mouth of the Atchafalaya river. This is one
of two deltas that are being creating in.. along the Louisiana coast by sediment that
is being siphoned off the Mississippi river by its tributary, the Atchafalaya . The Atchafalaya
and the wax lake outlet deltas are prominent features that are readily visible from satellite
images. These deltas that are located in the Atchafalaya Bay are unique in that they are
expanding in size and their wetlands are more stable than those farther to the east in the
Mississippi River birdsfoot and other basins, which are currently deteriorating due to lack
of sediment and due to subsidence. C. Vervaeke: Got 4 centimeters on “c”,
4 centimeters on the top and 3 centimeters on core “d”. Core “c” was also sectioned
into three separate sections. The top, then the middle, eleven, cause it had real sandy
and the bottom, bottom 2, because it was all organic below that, it seemed all organic
below that sandy layer.N. Khan: Finally I’ve found that sand… K. McKee: Ohh, that’s a good one. K. McKee: So, what we are doing here is collecting
samples of the surface layer of sediment and measuring the thickness of the deposit. We
also save the sample to take back to the lab for further analysis of the sediment characteristics. K. McKee: So, how deep was that? The top layer? N. Khan: Ah. In total it was around eleven. Narrator (McKee): Several centimeters of new
sediment were deposited in the freshwater marshes of the Atchafalaya basin. The presence
of plant roots marks the older soil layers. K. McKee: So as these plants get rooted, their
roots begin to bind the soil and hold it in place, preventing erosion. The stems also
slow the water, and this causes, when the water slows down, this causes ah sediment
particles to drop out of the water and deposit along the banks of this river. K. McKee: Now we’re headed east away from
the Atchafalaya basin and toward more saline marshes of the Terrebonne basin. As we move
away from the river, the influence of freshwater will decrease and we’ll may see less sediment
from this flood event. K. McKee: These are salt marshes dominated
by smooth cordgrass; the salinity here is much higher than in the fresher marshes we
just left. As we expected, there appears to be little sediment from the flood reaching
these marshes. C. Vervaeke: With one hand, push the corer
down and the other hand hold that. A. Constantin: How far down did you go? K. McKee: We are now headed toward the mouth
of the Mississippi River and the extensive freshwater marshes located in the Birdsfoot
delta. The high flow of water from the river here keeps these areas fresh and allows freshwater
plant species to colonize. The dominant plant you see here is Roseau cane. Narrator (McKee): The Mississippi River Birdsfoot
Delta, which is located at the terminus of the river, is so named because the shape created
by the river channels resembles a birdsfoot. C. Vervaeke: Push it down and pull up…the
piston up at the same time. And that’ll give us a good core. Come up and then we’ll
extrude the core using the piston. The top part, real loose, not many roots in it. So
there’s a distinct layer ending about right there. And the bottom is really, really compact…..
and not holding much water at all. Narrator (McKee): What controls marsh stability?
When marsh vegetation colonizes a mudflat, the sediment that created the mudflat settles
and compacts over time, causing subsidence; the soil surface then sinks and flooding increases.
Floodwaters, however, bring in more sediment, which settles out forming a new layer on the
soil surface. Plant roots bind the new sediment and also add organic matter. The soil surface
builds upward and thus counterbalances subsidence. K. McKee: So beneath my feet the ground is
slowly sinking. And if I stood here long enough, I would become submerged over time. But if
the rate of sedimentation on the soil surface balances the rate of compaction below the
surface, then the marsh elevation stays level with the prevailing water level. Narrator (McKee): And with sea-level rise,
even more sediment will be needed to avoid submergence of coastal wetlands. Narrator (McKee): So now we’ve completed
our survey of the coast and can plot our sampling stations using their GPS coordinates on this
map. The different colors of the stations show qualitatively how much sediment was deposited
based on our measurements in each location. We can see that more sediment was found near
the rivers, especially in the Atchafalaya basin, and less sediment reached those marshes
farthest from the rivers. Narrator (McKee): The information gathered
during this survey will help in understanding effects of river flooding on wetlands and
also to develop better ways to restore deteriorating wetlands in the Mississippi River delta.

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