Geoscience Reference
In-Depth Information
Figure 15-17.
Satellite image of the Mississippi River delta vicinity, 11 October, 1985. Major distributary channels:
1. Main Pass, 2. Pass á Loutre, 3. Southeast Pass, 4. South Pass, 5. Southwest Pass, and 6. Grand Pass. Based on
Landsat TM band 4 (near-infrared), which is particularly sensitive to emergent vegetation (light gray) and water
bodies (black). Image from NASA; processing by J.S. Aber.
slow but inexorable, and predictions are that
it will continue throughout the twenty-i rst
century for a total of 1-2 m increase in sea
level worldwide.
• Compaction of deltaic sediment - Clay and
silt deposits, which dominate the marshes,
bays and sounds of the delta region are ini-
tially quite porous with high water content.
As the sediment becomes buried, the water
is gradually expelled by increasing pressure,
the sediment compacts, and the land surface
subsides. This phenomenon happens in all
deltas; continuous deposition of new sedi-
ment is necessary to maintain the delta
surface.
• Hurricanes - Storm surges and strong waves
may erode shores rapidly, breach barrier
islands and shoals, and redistribute sediment
into lagoons and bays. This region has fre-
quent hurricanes. Grand Isle, Louisiana (Fig.
15-20), for example, has been struck by hur-
ricanes 31 times since 1851, a recurrence
interval of one hurricane about every i ve
years (Doyle 2009). The popular concept
that hurricanes are rare or unusual events is
simply wrong; hurricanes are part of the
natural regime of coastal Louisiana.
• Reduction in sediment supply - From their
headwaters in the Rocky Mountains, north-
ern Plains, and Appalachians all major rivers
and many lesser streams of the Mississippi
drainage basin are controlled by various
human structures (Fig. 15-21). This impact
extends to tens of thousands of smaller arti-
i cial ponds and retention basins on minor
tributaries (Fig. 15-22). The net result is that
much sediment is retained in upstream res-
ervoirs; the Mississippi Delta is starved of
sediment.
• Conversion of wetlands for agriculture -
Federal agricultural subsidies led to clearing
and draining of wetlands for conversion to
cropland, mainly for soybean and cotton
production, particularly from the 1950s to
