Page 408 - Environment: The Science Behind the Stories

CEntraL CaSe StUdy

Starving the Louisiana Coast

of Sediment

NORTH
AMERICA
“The Louisiana and Mississippi coastal region is
critical to the economic, cultural, and environmental
Mississippi River integrity of the nation.”
— Nancy Sutley, Chair of the White House Council
LOUISIANA on environmental Quality
Atlantic
Ocean
“What really screwed up the marsh is when they put
Gulf of
Mexico the levees on the river. They should take the levees
out and let the water run; that’s what built the
land.”

— Frank “Blackie” Campo, Resident of Shell
Pacific Beach, Louisiana
Ocean SOUTH
AMERICA

The state of Louisiana is shrinking. Its coastal wetlands straddle When waters become too deep, the vegetation dies, and soils
the boundary between the land and the ocean, and these wet- are then washed away by the ocean. The natural compaction is
lands are disappearing beneath the waters of the Gulf of Mex- offset, however, by inputs of sediments from the river and from
ico. Louisiana loses 65 km (25 mi ) of coastal wetlands each the deposition of organic matter from marsh grasses. These
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year, and comparisons of wetland area from the mid-1800s to additions keep soil levels high, water depths relatively stable,
the early 1990s show drastic losses (Figure 15.1a). Since the and vegetation healthy.
1930s alone, Louisiana has lost nearly 4900 km (1900 mi ) of So why are Louisiana’s wetlands being swallowed by
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coastal wetlands—an area roughly the size of Delaware. the sea? It’s because people have modified the Mississippi
Louisiana’s coastal wetlands transition from communities River so extensively that much of its sediments no longer
of salt-tolerant grasses at the ocean’s edge to freshwater bald reach the wetlands that need them. The river’s basin contains
cypress swamps further inland. These ecosystems support a roughly 2000 dams, which slow river flow and allow sedi-
diversity of animals, including eagles, pelicans, shrimp, oys- ments suspended in the water to settle in reservoirs. This not
ters, black bears, alligators, and sea turtles. The state’s coastal only prevents sediments from reaching the river’s delta, but
wetlands also protect cities such as New Orleans and Baton also slowly fills in each dam’s reservoir, decreasing its volume
Rouge from damaging storms. Vegetation in these wetlands and shortening its life span. Therefore, dams in Minnesota,
acts as a windbreak on strong winds and as a water break on Montana, and Pennsylvania and other locations throughout CHAPTER 15 • Fr E shwat E r s yst E m s and rE sour CE s
waves coming inland from the Gulf. the Mississippi basin affect the Louisiana coastline hundreds
Louisiana’s millions of acres of coastal wetlands were of miles downriver.
created over the past 7000 years as the Mississippi River The Mississippi River is also lined with thousands of miles
fanned out and deposited its sediments at its delta before of levees (long, raised mounds of earth). These structures pre-
emptying into the Gulf of Mexico. The Mississippi River accu- vent small-scale flooding, and the mouth of the Mississippi is
mulates large quantities of sediment from water flowing over lined with levees to provide a deep river channel for shipping
land into streams in the river’s 3.2-million-km (1.2-million-mi ) into the Gulf of Mexico. These levees prevent the river from
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watershed (Figure 15.1b). Much of this sediment originates fanning out into its delta and turn the lower Mississippi into a
from the Missouri River basin that drains America’s agricul- “barrel” that shoots sediments off the continental shelf into the
tural heartland. deep waters of the Gulf (Figure 15.1c).
The salt marshes in the river’s delta naturally compact over Although Louisiana’s economy has benefited from oil and
time. This compaction lowers the level of the marsh bottom gas extraction, these activities have also promoted wetland
and submerges vegetation under increasingly deeper waters. losses. The extraction of large quantities of oil, natural gas, 407

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