Geoscience Reference
In-Depth Information
water and diversion of surface water. The net
result is diminished l ow of surface water
throughout the Everglades drainage system with
many environmental consequences. Periodic
i res, for example, are ignited by lightning and
help to maintain the grassland habitat by limit-
ing invasion of woody brush and trees. Prior to
human intervention, these i res swept lightly
over the marshes, and the sawgrass sprouted
more vigorously following i res. However, drain-
ing the Everglades and i re suppression have
led to a new burning regime, in which i res
burn deeply, destroy roots and underlying
peat, and even penetrate into hammocks
(Niering 1985).
Surface water in the Everglades is naturally
nutrient-poor. However, upstream agricultural
runoff delivers large quantities of fertilizer,
namely nitrogen and phosphorus, to the Ever-
glades. Where this has occurred, sawgrass prairie
has converted into cattail marsh, with deleteri-
ous effects on organisms and water quality. One
result of higher phosphorus concentration is
increased methanogenesis (Castro, Ogram
and Reddy 2004). Recognizing these negative
impacts, the State of Florida initiated a multimil-
lion dollar
Save our Everglades
program in 1983.
However, the state and federal governments
came into conl ict in 1988, when the federal
government sued the State of Florida and the
South Florida Water Management District
because of their failure to protect ecosystems in
the Loxahatchee National Wildlife Refuge. The
state approved the Everglades Forever Act in
1994, and the federal Comprehensive Everglades
Restoration Plan in 2000 is the largest environ-
mental recovery project ever in the United States
(Dugan 2005). Full implementation will require
billions of dollars and decades of work, much
of which remains to be done.
small portion of Canada. All these river systems
are heavily modii ed and managed from their
headwater sources to the Gulf of Mexico for
navigation, l ood control, hydropower, water
supply, and recreation. The term “delta” is com-
monly applied to Mississippi bottomland from
Memphis, Tennessee southward; however, this
discussion focuses on the coastal region of
southern Louisiana wherein freshwater drain-
age systems grade into marine conditions of
the Gulf of Mexico (Fig. 15-16). Louisiana con-
tains 40 percent of all wetlands in the contigu-
ous United States and contributes 30 percent of
the nation's seafood (Whigham et al. 2010),
which emphasizes the extreme importance
of the Mississippi Delta and adjacent coastal
environments.
The Mississippi Delta was built during the
past several millennia as postglacial sea level
rose and sediment was washed down from the
continental interior. Many times, delta lobes
have shifted as the river meandered back and
forth across the low-lying coastal region between
the Atchafalaya Bay and Mississippi Sound (see
Color Plate 8-13). The modern delta is a bird-
foot delta, so-called because of the way the
distributary channels branch out at Head of
Passes (Fig. 15-17). Between the early 1800s and
1985, numerous changes took place in the
dynamic delta complex (Fig. 15-18). Among the
major developments are ini lling of Bay Ronde
and Garden Island Bay, the appearance of Main
Pass, extension of Pass á Loutre, and reductions
of Southeast, South and Southwest passes.
During the past two decades, however, the
lower delta complex has lost considerable area
(Fig. 15-19). Among the major losses are the
Breton and Chandeleur islands, which have
nearly disappeared, and reductions of Pass á
Loutre and Southeast Pass, whereas South Pass
has gained land.
The current paths of the Mississippi River
and its distributary channels across the delta are
maintained by an extensive network of levees
and channel dredging, and the river is coni ned
upstream for more than 1000 km of its course
(Dugan 2005). Left to its own devices, the Mis-
sissippi would have breached its channel
upstream and followed a shorter, steeper route
15.5.2 Mississippi River delta
The delta of the Mississippi River is the end
point of one of the world's largest terrestrial
drainage systems, which includes the Missis-
sippi, Ohio, Tennessee, Missouri, Platte, Arkan-
sas, and other major rivers draining the
continental interior of the United States and a
