Estuary Processes and Managment - River Dynamics and Integrated River Management - page 467

The Mississippi River mouth extends deeply into the Mexico Gulf. Avulsions occurred in the Mississippi
delta along the coast of Louisana as successive channels searched for higher gradient than their precursors
(Leeder, 1983). Delta development is affected or partly controlled by humans. Upstream reservoirs, changes
in agricultural practices and land uses, and bank stabilization measures have reduced average sediment
loads in the lower Mississippi River by approximately 67% since the 1950s (Kesel, 1988). The average
concentration of suspended sediment reduced from 0.8 kg/m 3 in 1950 to 0.24 kg/m 3 in 2000. Moreover,
the natural avulsion from the present Mississippi River channel to the Atchefalaya channel has been
stopped by human structures. The present delta is eroded by waves and tidal current. The delta lobe has
become slender and thin.
The Nile River flows 6,825 km from central Africa to the delta and is the longest river in the world,
which is the reason for the existence of Egypt and the great Egyptian civilization. It is an exotic river
with no perennial-flow tributaries, but ephemeral wadis contribute sediment and water infrequently. In
contrast to other large rivers, the Nile is straight, and appears to be relatively stable. The Nile today has
been described as a "very low energy river with little capability to erode its banks (only 12% of its banks
are experiencing erosion) or change its channel" (Mercer et al., 1992). In the past centuries the Nile River
was a male river with annual sediment load about 160 million tons. The sediment concentration was as high
as about 3.5 kg/m 3 . A fan-shape delta developed with two channels extending into the red sea (Stanley
and Winkley, 1994). The delta, with an area of about 23,300 km², has been formed through deposits over
the course of tens of thousands of years on the originally shallow seabed. The area around the Nile has
about 34 million inhabitants, which equates to half the entire population of Egypt. The coastline is about
400 km long, from Alexandria in the west to Port Said and the outlet of Suez Canal in the east. The
construction of the Aswan Dam and the High Aswan Dam in addition to many barrages has changed
the river remarkably. The river's long-term annual peak discharge of 8,430 m 3 /s has been reduced to a
maximum release of 2,550 m 3 /s, while the suspended sediment concentration in flood season is now only
about 0.1 kg/m 3 (Stanley and Winkley, 1994). These changes affect the delta development and coastal
erosion of the delta. At the present, there are several brackish lagoons or lakes, of which Manzala and
Burullus are the largest. The delta has two main channels: the Rosetta and Domietta rivers. The sediment
load has been reduced by 99% due to dams and barrages. Thus, the delta has changed from male to female.
A stable channel web consisting of numerous distributaries and canals has been formed.
The sediment load reduction also caused the Nile delta to change from a sediment deposition center
into a man-altered coastal plain. The delta has stopped spreading out into the Mediterranean and locally
is receding. It is no longer an active natural delta. Very little sediment is presently carried seaward to
replenish the delta coast. The Nile has transformed from a seasonally fluctuating fluvial regime to a
year-round storage and regulated flow system (Howell and Allan, 1994). The extended length of the river
mouth has been shortened by 8 km in the past 80 years. The land loss was caused not only by wave
current and dams and barrages, but also by retention of sediment on the delta plain due to channelization,
irrigation and land reclamation on the delta proper (Stanley, 1996).
8.1.3 Ecology of Male and Female Rivers
In general, a river changes its gender from male to female due to sediment load reduction. Sharp sediment
load reduction reduces land creation and causes problems to estuary management. For instance, sediment
load reduction in the Yellow River has stopped land creation and resulted in land loss due to coastal
erosion. In the Mississippi delta sediment load reduction has resulted in wetland loss. Nevertheless, if a
gender change does not occur too fast and the delta has no severe pollution, the ecology may improve and
the biodiversity of both fish and macro-invertebrates will increase after the gender change.