Environmental Engineering Reference
In-Depth Information
Mississippi delta during these decades clearly refl ect changes in sediment
loads arriving at the mouth of the Mississippi, which drastically reformed
the structure and functions of associate coastal wetland environments
(Wells 1996). The earliest maps of the Mississippi River delta show channels
bounded by narrow marshes, but in the mid-1800s a period of rapid land
formation began, resulting in 560 km 2 of new land in 100 yr (Wells and
Coleman 1987). Maps of the 1930s delta show large wetland areas, much
of which have been more recently eroded.
There seems to be a common pattern in which, as agricultural land
use intensifi es, sediment loads carried by rivers draining the area increase.
As it becomes necessary to strengthen agricultural production, water
interception works are built, and sediment loads entering coastal waters
decrease. The rivers discussed here are examples that demonstrate how
anthropogenic changes in sediment transport have become the dominant
factor determining whether relative sea level rise is positive or negative
in many deltas (Syvitski and Kettner 2011). These well known cases may
be used as indicators of what may occur in other developing watersheds
around the world. Because of the positive relationship between suspended
sediments and land area in these deltas, it may be inferred that future
variation in suspended sediments delivered to the mouth of rivers, higher
or lower, would likely lead to proportional wetland changes in the coastal
zone.
Species Introduction
Wetland plants have been introduced to coastal areas worldwide, to make
use of their commonly high productivity, tolerance to inundation, and
peat-building capabilities. S partina anglica is a common halophyte in lower
marshes of northern Europe. This fertile allopolyploid arose by the end
of the 1880s in Southampton Water, UK, by chromosomal doubling from
Spartina townsendii , a hybrid between the introduced Spartina alternifl ora
and the native Spartina maritime (Ayres and Strong 2001). Although S.
anglica invaded large areas during the fi rst 30 years after being reported
(Raybould 1997), during the late 1920s and early 1930s the species began
to show signs of loss of vigor at some locations, leading to widespread
diebacks. This process of recession, fi rst described by Goodman et al. (1959),
has continued up to the present. However, the concept of comprehensive
spread followed by sudden decline cannot be generalized, and there are
some locations where S. anglica has maintained its vigor and capacity for
colonization of mudfl ats, displacing eel grass ( Zostera marina ) and algae
(Raybould 1997).
In addition to a rapid natural spread following its appearance, S.
anglica was also extensively planted to stabilize soft sediments, resulting
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