Deltas and human impacts Deltaic settings continue to
play an important role in global society. As mentioned earlier, deltaic environments served as the culture hearth for
early civilizations throughout the world because of their
tremendous variety of food resources such as fish and
wildlife. Moreover, the rich alluvial soils of deltaic landscapes allowed for the establishment of bountiful crops
vital to the establishment and expansion of early cultures
(Stanley and Warne, 1997). The historic impact of human
development on delta evolution is revealed in a comprehensive study of European deltas by Maselli and Trincardi
(2013). They showed that the Ebro, Rhone, Po, and Danube deltas underwent rapid progradation during the
Roman Empire as a result of widespread deforestation
and extensive agricultural use of the land, leading to
greater sediment contribution to rivers and delta growth.
They also reported widespread delta erosion and shoreline
retreat coincided with the collapse of the Roman Empire in
400 AD when lands were reforested leading to lower sediment influx to rivers. A second period of broad delta erosion was caused by extensive dam building between
1960–1990 (Maselli and Trincardi, 2013). Similar comprehensive studies and findings have also been reported
for the Mississippi River delta (Kessel, 2003; Tweel and
Turner, 2012) and Ebro (Guillen and Palanques, 1997).
The extent of delta erosion on a global scale is summarized in a thoughtful paper by Giosan et al., 2014. They
show that dam construction, levee building, and other
human impacts have led to a drastic reduction in sediment
conveyance to many of the world’s largest deltas including the Nile, Indus, Mississippi, Danube, Yangtze, and
Rhone with sediment loads having decreased by
60–98 % during the past century. They state that most of
the world’s largest and moderately sized deltas have experienced significant erosion and are predicted to succumb
to rising sea-level during the next century, as they do not
receive sufficient sediment to maintain their elevation
and areal extent (Giosan et al., 2014). The demise of deltaic lands can be hastened by subsidence brought on by
the subsurface withdrawal of hydrocarbons (e.g., Mississippi; Morton and Bernier, 2010) or water (e.g., Po;
Teatini et al., 2011). A detailed analysis by Blum and Roberts (2009) of the Mississippi River sediment load indicates that the amount of sediment coming down the river
is vastly inadequate to maintain the existing delta plain
above sea-level, and therefore, they predict that most of
the delta will drown by 2100. Contributing to erosion
and drowning, it also should be noted that 11 of the largest
25 deltas are impacted by hurricanes (cyclones, typhoons)
(Bianchi and Allison, 2009) and that 85 % of the world’s
largest deltas have experienced severe flooding during
the last two centuries (Syvitski et al., 2009).
The partial solution to erosion and the eventual drowning of deltas will require a multiple-faceted approach
undoing certain human alterations and implementing
new ones. For example, dam removal will allow sediment
behind dams to be transported to the coast, reestablishing
natural sediment conveyance systems. Since 1912, 1,150
dams have been removed along American rivers, including 51 dams in 2013 (American Rivers, 2013). Other solutions to delta land loss might involve: (1) initiating
crevasse splays in the lower delta plain, (2) creating channel diversions to fill large open water areas (e.g., Atchafalaya delta formation, Mississippi delta), or (3) diverting
the entire river flow to form new distributary delta lobes.
Summary and conclusions
Deltas are sedimentary deposits that exist at the mouths of
rivers and whose morphology and facies architecture are a
product of riverine sediment discharge (type and load) and
processes within the receiving basin, including waves,
tides, storms, and tectonic regime. Modern deltas began
forming 6–8,000 years ago coincident with a slowing in
the rate of SLR when river-derived sediment filled estuaries producing shoreline progradation. Riverine-dominated
deltas are characterized by elongate sandy distributary
deposits with intervening muddy bays and wetlands;
wave-dominated deltas tend to be cuspate to lobate in
shape having numerous prograding chenier/beach ridges;
tide-dominated deltas are characterized by funnel-shaped
embayments with linear islands and/or subtidal sand
shoals and onshore mud flats and wetlands. Deltas of the
world are under siege due to their overall low-lying elevation and high rates of relative SLR brought on by subsidence and accelerating global SLR. In addition, dams
have drastically reduced sediment contribution to most
deltas, leading to widespread shoreline erosion. The fact
that deltas coincide with dense human population centers
means that they are particularly susceptibility to RSLR,
especially because many deltas are impacted by periodic
cyclones and severe flooding.
Bibliography
Alonso, B., Field, M. E., Gardner, J. V., and Maldonado, A., 1990.
Sedimentary evolution of the pliocene and pleistocene Ebro margin, northeastern Spain. Marine Geology, 95, 313–331.
American Rivers, 2013. American Rivers Strategic Plan
2014–2018, Washington, DC, 13 pp. Accessed at http://www.
americanrivers.org/assets/pdfs/AmericanRiversStrategicPlan20142018.pdf?506914.
Bates, C. C., 1953. Rational theory of delta formation. American
Association of Petroleum Geologists Bulletin, 37, 2119–2161.
Bianchi, T. S., and Allison, M. A., 2009. Large-river delta-front
estuaries as natural “recorders” of global environmental change.
Proceedings of the National Academy of Sciences, 106,
8085–8092.
Blum, M. D., and Roberts, H. H., 2009. Drowning of the Mississippi Delta due to insufficient sediment supply and global
sea-level rise. Nature Geoscience, 2, 488–491.
Boyd, R., Suter, J. R., and Penland, S., 1989. Sequence stratigraphy
of the Mississippi Delta. Gulf Coast Association of Geological
Societies Transactions, 39, 331–340.
Canestrelli, A., Fagherazzi, S., Defina, A., and Lanzoni, S., 2010.
Tidal hydrodynamics and erosional power in the Fly River delta,
Papua New Guinea. Journal of Geophysical Research, Earth
Surface, 115, F04033. 14 pp.
180
DELTAS
play an important role in global society. As mentioned earlier, deltaic environments served as the culture hearth for
early civilizations throughout the world because of their
tremendous variety of food resources such as fish and
wildlife. Moreover, the rich alluvial soils of deltaic landscapes allowed for the establishment of bountiful crops
vital to the establishment and expansion of early cultures
(Stanley and Warne, 1997). The historic impact of human
development on delta evolution is revealed in a comprehensive study of European deltas by Maselli and Trincardi
(2013). They showed that the Ebro, Rhone, Po, and Danube deltas underwent rapid progradation during the
Roman Empire as a result of widespread deforestation
and extensive agricultural use of the land, leading to
greater sediment contribution to rivers and delta growth.
They also reported widespread delta erosion and shoreline
retreat coincided with the collapse of the Roman Empire in
400 AD when lands were reforested leading to lower sediment influx to rivers. A second period of broad delta erosion was caused by extensive dam building between
1960–1990 (Maselli and Trincardi, 2013). Similar comprehensive studies and findings have also been reported
for the Mississippi River delta (Kessel, 2003; Tweel and
Turner, 2012) and Ebro (Guillen and Palanques, 1997).
The extent of delta erosion on a global scale is summarized in a thoughtful paper by Giosan et al., 2014. They
show that dam construction, levee building, and other
human impacts have led to a drastic reduction in sediment
conveyance to many of the world’s largest deltas including the Nile, Indus, Mississippi, Danube, Yangtze, and
Rhone with sediment loads having decreased by
60–98 % during the past century. They state that most of
the world’s largest and moderately sized deltas have experienced significant erosion and are predicted to succumb
to rising sea-level during the next century, as they do not
receive sufficient sediment to maintain their elevation
and areal extent (Giosan et al., 2014). The demise of deltaic lands can be hastened by subsidence brought on by
the subsurface withdrawal of hydrocarbons (e.g., Mississippi; Morton and Bernier, 2010) or water (e.g., Po;
Teatini et al., 2011). A detailed analysis by Blum and Roberts (2009) of the Mississippi River sediment load indicates that the amount of sediment coming down the river
is vastly inadequate to maintain the existing delta plain
above sea-level, and therefore, they predict that most of
the delta will drown by 2100. Contributing to erosion
and drowning, it also should be noted that 11 of the largest
25 deltas are impacted by hurricanes (cyclones, typhoons)
(Bianchi and Allison, 2009) and that 85 % of the world’s
largest deltas have experienced severe flooding during
the last two centuries (Syvitski et al., 2009).
The partial solution to erosion and the eventual drowning of deltas will require a multiple-faceted approach
undoing certain human alterations and implementing
new ones. For example, dam removal will allow sediment
behind dams to be transported to the coast, reestablishing
natural sediment conveyance systems. Since 1912, 1,150
dams have been removed along American rivers, including 51 dams in 2013 (American Rivers, 2013). Other solutions to delta land loss might involve: (1) initiating
crevasse splays in the lower delta plain, (2) creating channel diversions to fill large open water areas (e.g., Atchafalaya delta formation, Mississippi delta), or (3) diverting
the entire river flow to form new distributary delta lobes.
Summary and conclusions
Deltas are sedimentary deposits that exist at the mouths of
rivers and whose morphology and facies architecture are a
product of riverine sediment discharge (type and load) and
processes within the receiving basin, including waves,
tides, storms, and tectonic regime. Modern deltas began
forming 6–8,000 years ago coincident with a slowing in
the rate of SLR when river-derived sediment filled estuaries producing shoreline progradation. Riverine-dominated
deltas are characterized by elongate sandy distributary
deposits with intervening muddy bays and wetlands;
wave-dominated deltas tend to be cuspate to lobate in
shape having numerous prograding chenier/beach ridges;
tide-dominated deltas are characterized by funnel-shaped
embayments with linear islands and/or subtidal sand
shoals and onshore mud flats and wetlands. Deltas of the
world are under siege due to their overall low-lying elevation and high rates of relative SLR brought on by subsidence and accelerating global SLR. In addition, dams
have drastically reduced sediment contribution to most
deltas, leading to widespread shoreline erosion. The fact
that deltas coincide with dense human population centers
means that they are particularly susceptibility to RSLR,
especially because many deltas are impacted by periodic
cyclones and severe flooding.
Bibliography
Alonso, B., Field, M. E., Gardner, J. V., and Maldonado, A., 1990.
Sedimentary evolution of the pliocene and pleistocene Ebro margin, northeastern Spain. Marine Geology, 95, 313–331.
American Rivers, 2013. American Rivers Strategic Plan
2014–2018, Washington, DC, 13 pp. Accessed at http://www.
americanrivers.org/assets/pdfs/AmericanRiversStrategicPlan20142018.pdf?506914.
Bates, C. C., 1953. Rational theory of delta formation. American
Association of Petroleum Geologists Bulletin, 37, 2119–2161.
Bianchi, T. S., and Allison, M. A., 2009. Large-river delta-front
estuaries as natural “recorders” of global environmental change.
Proceedings of the National Academy of Sciences, 106,
8085–8092.
Blum, M. D., and Roberts, H. H., 2009. Drowning of the Mississippi Delta due to insufficient sediment supply and global
sea-level rise. Nature Geoscience, 2, 488–491.
Boyd, R., Suter, J. R., and Penland, S., 1989. Sequence stratigraphy
of the Mississippi Delta. Gulf Coast Association of Geological
Societies Transactions, 39, 331–340.
Canestrelli, A., Fagherazzi, S., Defina, A., and Lanzoni, S., 2010.
Tidal hydrodynamics and erosional power in the Fly River delta,
Papua New Guinea. Journal of Geophysical Research, Earth
Surface, 115, F04033. 14 pp.
180
DELTAS