WHAT IS THE CONNECTION BETWEEN THE GEOLOGY AND THE HISTORY
OF NEW ORLEANS?
To understand how Hurricane Katrina came to be so damaging to New Orleans you
need to know a little bit of geology and a little bit of history.
Start with the geology: When the Muddy Mississippi floods, it washes over its banks,
dropping its heaviest sands and silts closest to its channel. Flood after flood it builds a
natural levee to an average height of fifteen feet above the adjacent floodplain. Relieved of
the heavy sands, the floodwaters wash a carpet of fine silts across the low-lying floodplain
and create a poorly drained, marshy backswamp in which cypress and tupelo flourish.
Now a little bit of history: In 1718 Sieur de Bienville founded New Orleans on the
natural levee of the Mississippi north of its mouth. Over the objections of engineers he laid
out the town on the left bank of the river at the point where the river swings nearest to
Bayou St. John and Lake Pontchartrain to the north. Residents built their houses on high
ground, on the natural levee of the river and near the head of Bayou St. John, which gave
them a shortcut to the Gulf of Mexico through Lake Pontchartrain. A cypress forest fronted
by a marsh, the backswamp, lay between the village and the lake and protected the village
from storm surges in the lake. New Orleans held to the natural levee until the 1920s when
developers cleared the cypress forest, drained the marsh, reclaimed land from the lake, and
built a floodwall to keep the lake from flooding the neighborhoods that followed. The newly
drained land compacted and sank below sea level.
HOW DOES A RIVER BUILD A DELTA AND WHAT DOES THAT HAVE TO DO
WITH KATRINA AND NEW ORLEANS?
When a fast moving stream meets a still body of water, it almost comes to a halt and
deposits its load of sediment in the still body of water. In this way the Mississippi built its
delta.
And: When the river becomes too long and its slope to the sea too flat, it finds a
shorter, steeper route to the sea. The Mississippi, south of Old River at the head of the
Atchafalaya, has done this at least five times in the last 7,000 years. First it deposited its
sediment on the western edge of the delta, where Hurricane Rita touched in mid-September
2005. Then it shifted to the eastern side of the delta, southeast of New Orleans, where
Hurricane Katrina tore up the landscape in August 2005. Then the river shifted directly
south of New Orleans. Then, a little to the west of there, and finally, about 1300 years ago, it
began to deposit the modern delta, laying it down between two older deltas and contributing
sediment to them also.
More delta building: A distributary channel is one that breaks off from the trunk
channel of a river and carries its sediment to the sea. Such a channel functions much like a
trunk channel. Like the trunk channel, it builds a natural levee, adding to it with each flood.
Like the trunk channel, it washes fine silts and clays over its natural levee into a poorly
drained backswamp. And, it can split into a splay of branching channels: each one pushing
into the Gulf, each one forming a natural levee, each one washing fine silts and clays into a
poorly drained marsh. Together they form a delta lobe laced together by a network of
natural levees.
Basins, which form between the branching channels, fill with marshes--saline
marshes closest to the delta front, freshwater inland swamps farthest away from the delta
front. In between, the marshes progress from freshwater to brackish to saline as the lobe
pushes out into the Gulf of Mexico.
WHAT IS SUBSIDENCE AND WHAT DOES THAT HAVE TO DO WITH KATRINA
AND NEW ORLEANS?
Finally, there is a natural process called subsidence: As long as a distributary
continues to deliver sediment to its delta front, and as long as flood deposits feed the basins
between the separate channels, mud flats form in the basins and are quickly colonized by
fresh water vegetation. Once the distributary ceases to deliver sediment to its delta front,
once flood deposits no longer feed the basins between separate channels, and once the cycle
of growth and decay in the marsh can no longer sustain the marsh, sea water seeps in and
begins to eat away at the marsh, replacing it with shallow lakes or bays.
HOW DOES SUBSIDENCE INCREASE THE INTENSITY OF HURRICANES?
There is a rule of thumb: Every mile or two of marshland reduces a storm surge
from a hurricane by as much as a foot. At the beginning of the twenty-first century Louisiana
was disappearing, subsiding, at the rate of one acre every twenty-four minutes. Project that
rate over a year, and Louisiana was disappearing at the rate of 20,000-25,000 acres per year,
which correlates to twenty-five to thirty-five square miles per year. In the eighty years before
2000 Louisiana lost 600,000 acres of vegetated wetlands. If this continued, the Louisiana
shoreline would creep inland some thirty-three miles in some areas of the coast. Louisiana
was disappearing and New Orleans was threatened by hurricanes, as coastal marshes
receded.
WHAT CHANGES TO THE MISSISSIPPI AND ITS DELTA AGGRAVATED THE
THREAT TO NEW ORLEANS BY HURRICANES?
The process started with the first levee built at New Orleans in 1723. Every change
we have made since to the natural functions of the river has decreased the amount of
sediment delivered to the Gulf of Mexico for the construction of coastal marshes.
James Eads’ jetties at the mouth of the river may have allowed the river to cut a
thirty-foot navigation channel through the sandbar blocking the South Pass of the modern
delta, but delivered sediment carried by the river to very deep water at the continental shelf,
where it washed away, never to be used for marsh building. Closure of the old distributaries
of the river may have prevented flooding in the bayou country of Louisiana, but cut the flow
of Mississippi freshwater sediment to the coastal marshes. Revetments built of concrete mats
may have stabilized the navigation channel, but reduced erosion of the banks, a source of
sediment in the marshes. Artificial levees, extending clear to the Gulf of Mexico, may have
made human habitation of the Mississippi delta south of Cape Girardeau possible, but
prevented the Mississippi from refreshing Louisiana’s swamp and marshes with freshwater
and sediment when it did flood. Channel dams may have made navigation on the Upper
Mississippi economically feasible, but they retained its sediment north of Alton, Illinois.
Dams on the Missouri, from which the Mississippi drew sixty percent of its sediment, did
the same. In short at the beginning of the twenty-first century, the coastal marshes south of
New Orleans received eighty percent less sediment than they had at the beginning of the
twentieth and had been servely eroded away. And, they became dependent on rain for
freshwater.
HOW DID OIL DRILLING IN THE MARSHES CAUSE THEIR DESTRUCTION?
The destruction of the marshes was further aggravated by the discovery of oil in the
early twentieth century. Oil companies dredged long, straight navigation and exploration
canals--an urban-like grid laid over the meandering bayous that laced the coastal marshes
together. They dropped their dredge material along the edge of their canals, created spoil
banks, levees that disrupted the natural flow of water across the marshes. They prevented
water and sediment from refreshing the marshes; they created open lakes. Each vessel
passing through the canals dragged a wake behind it, which spread out and eroded the
banks, creating still more open water. Finally, the canals broke down the zone between fresh
water and salt water, allowing salt to bleed into fresh water marshes, killing the native plants
and trees, forcing wildlife to adapt or go elsewhere.
WHAT ABOUT THE MISSISSIPPI RIVER GULF OUTLET?
The Mississippi River Gulf Outlet barge canal may have shortened the distance
between the Port of New Orleans and the Gulf of Mexico by 40 miles, but its construction
decimated the marshes southeast of New Orleans—marshes that would have absorbed the
brunt of hurricanes like Katrina. On August 29 MR GO, as the locals call it, funneled
Katrina’s storm surge into its shared channel with the Gulf Intracoastal Waterway, into the
heart of New Orleans, where the surge breached the floodwalls of the Industrial Canal and
flooded the lower ninth ward in the eastern part of the city.
WHAT ARE THOSE OTHER CANALS THAT CAUSED THE FLOODING OF NEW
ORLEANS?
Because the city lies in a bowl surrounded by levees, every drop of rain that falls on
New Orleans must be pumped out through drainage canals built at the beginning of the
twentieth century. The design and/or the construction of the floodwalls along the 17th street
and London drainage canals were flawed. Rather than overtopping the floodwalls along the
canals, Katrina’s storm surge undermined them. They collapsed, flooding the rest of the city,
all but the French Quarter, which was built on the natural levee.
WHAT IS THE VALUE OF LOUISIANA’S MARSHES AND CAN THEY PROTECT
NEW ORLEANS AND OTHER COASTAL CITIES?
Louisiana’s coastal wetlands buffer storms. One to two miles of marshland can
reduce a storm surge by one foot. The marshes absorb nutrients, sediment, and
contaminants. They serve as breeding, spawning, feeding, and nursery grounds for fish and
shellfish, which migrate to their freshwater nurseries in the summer, and return to the Gulf
when temperatures drop in the fall. Migratory birds rest on Louisiana’s barrier islands on
their annual migration from Central and South America. Waterfowl, wading birds, and
shorebirds colonize its freshwater marshes, attracted by their diverse menu of fish, shellfish,
and vegetation. The marshes provide habitat for the endangered brown pelican and seventy
pairs of bald eagles. Fur bearers—nutria, muskrat, mink, raccoon, otter, bobcat, beaver,
coyote, and opossum—thrive in the marshlands. The American alligator—once endangered,
now abundant—nests along the their banks.
Louisiana’s commercial fishermen harvest 1.1 billion pounds of fish and shellfish, up
to sixteen percent of the nation’s catch, valued at 1.5 billion dollars a year to Louisiana’s
economy. Recreational anglers contribute another 235 million dollars. More than forty
percent of the nation’s fur harvest comes from Louisiana. Alligator meat and hides bring in
thirty million a year. Twenty to twenty-five percent of the nation’s oil and natural gas supply
moves through Louisiana. Four hundred million tons of goods move through the Port of
New Orleans.
Louisiana’s three million acres of coastal wetlands are more productive than many
agricultural lands. If the loss of wetlands continues, commercial and recreational fishing will
likely decline by thirty percent, putting fifty thousand jobs at risk. Migratory birds and furbearers that depend on the marshes will decline. The cost of treating drinking water will rise,
along with the cost of salt and other minerals taken from the coast. Alligator meat will once
again become a delicacy.