Marine Deltas

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nearshore wave climate and often destructional with well-developed strand plains as typical for the Sao Francisco Delta in Brazil. Lobate deltas (Figs. 5.86, 5.90–5.93), exemplified by the Niger Delta in Africa, are intermittent between con-structional and decon-structional forms and often possess a smooth coastal outline. Deltas shaped predominantly by tides may develop inlets and distributary channels that are straight and ori-entated parallel to the tidal movements as is the case in the Ganges-Brahmaputra or the Amazon deltas. “Truncated” deltas also occur (Fig. 5.89), with almost no advance seawards of a strait shoreline.

Some complex deltas have a long and compli-cated history. As a delta grows in one direction for some hundreds or thousands of years, it then breaks out to form a new distributary channel and begins to grow into the sea in another direction.

For example, the Mississippi River has built a series of overlapping deltas of different Holocene ages which over the past 6000yrs shifted both to the east and the west.

The world’s great deltas and associated wet-lands hold rich, fertile soils constituting prime agricultural lands with similar abundance of food resources in the adjacent marine areas. Today, they are home to millions of people as is true for Egypt’s Nile Delta, China’s Yangtze and Huang He deltas, the Mississippi delta on the Gulf Coast of the United States or the Ganges Delta of Bangladesh. Delta environments with their rich soils and the abundance of game and fish food resources allowed also for the establishment and expansion of prehistoric cultures and served as the cultural hub for ancient civilisations. Our modern technologically driven society explored

Fig. 5.86 Part of the Shatt-el-Arab, the joint delta of Euphrates and Tigris rivers in the Persian Gulf at 30° 05´N and 48° 48´E, 24 km wide. In this desert environment dry intertidal flats are typical. (Image credit: © Google Earth 2010).

Fig. 5.87 A simple triangular delta form at the west coast of Africa (Gulf of Guinea) at 3° 34´N and 9° 41´W. Width of image is 32 km. (Image credit: © Google Earth 2010).

Fig. 5.88 This cuspate delta at the north coast of the Yucatan Peninsula of Mexico shows continuous progradation with beach ridge sequences at 18°32´N and 92°39´W. (Image credit: © Google Earth 2010).

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and exploited ancient deltas as they often contain a wealth of fossil fuels such as hydrocarbons or coals. In many areas of the world, deltas have suffered from human activities: Since the 1930’s engineers have built flood controlling structures and dams along the river course and decreased the amount of sediment delivered to the delta thereby also preventing the small but frequent floods that nourish the delta wetlands in a natural state. Worldwide over 41,000 large dams are in operation in addition there are also many smaller dams and water reservoirs. Together, they block 14 % of the total global river flow, as well as enormous volumes of sediment (Syvitskivet al., 2009). This anthropogenic induced change adds to the process of subsidence in deltas: As we have seen, deltas grow with the addition of sediment, and they sink as the sediment becomes

com-pacted and the crust subsides under the weight of the sediment load. Today, entire cities such as Venice, built on a delta in Northern Italy or Bangkok, Thailand’s sprawling capital of more than 10 million people (once dubbed “Venice of the East”) built on the banks of the Chao Phraya River delta are sinking. The situation is wors-ened with groundwater, oil or natural gas being pumped out (both legally and illegally) to supply residential and industrial demands. A recent sci-entific study of 33 major river deltas in the world revealed that 24 of them are sinking (Syvitskivet al., 2009). The prospect of a watery future for these cities is amplified by the recent sea level rise. The future of deltas will depend on our wisdom and ability to balance environmental management strategies and conservation versus economic development.

Fig. 5.89 The beach ridges of this truncated delta in Mexico at 18° 37´N and 92° 27´W show that it formerly reached farther into the sea (Central America). The reason for delta erosion maybe subsidence, stronger waves/storms, lack of sediments, or a com-bination of all these factors. Width of image is 25 km. (Image credit: © Google Earth 2010).

Fig. 5.90 This debris fan at the east coast of Baja California (Mexico) is also a delta. 30° 15´N and 114° 39´W, image 12km wide. (Image credit: © Google Earth 2010).

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Fig. 5.91 A rounded delta at the south coast of Iran in the sheltered Persian Gulf at 26° 49´N and 55° 30´E. (Image credit:

© Google Earth 2010).

Fig. 5.92 116 The complex Danube delta in the Black Sea.

North-south extension is about 150 km. (Image credit: © Google Earth 2010).

Fig. 5.93 Arctic deltas a) at the northwest coast of Alaska at about 63°10´N and 164°10´W with a width of about 40 km. The complex pattern of channels in the delta result from an ever changing strong run- off in the short time of melt water discharge, which may last only some weeks per year. Inspite of a rather dry climate, impermeable permafrost makes a swampy environment with a lot of ponds and open water arms. (Image credit: © Google Earth 2010). (b) Arctic delta. Northern Russia at 73°24´N and 127°14´E with a width of 50 km. (Image credit: © Google Earth 2010).

Fig. 5.94 A Russian delta into the northern Black Sea at 46° 31´N and 32° 22´E, image is 25km wide. This is an example that deltas may even appear in the inner parts of bays, like the la Plata delta in the “Rio de la Plata” in northern Argentina. (Image credit:

© Google Earth 2010).

Fig. 5.95 Typical meandering tidal creeks in mangrove for-ests in tropical deltas of very flat lowlands and a shallow sea (Ganges-Brahmaputra, India, at 21°54´N and 89°45´E, 30 km wide). (Image credit: © Google Earth 2010).

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Fig. 5.98 Main delta mouths of the Mississippi birdfoot delta (29° 09´N and 89° 03´W), exhibiting natural levees along the outer river courses as narrow elevated banks. Width of the scene is about 35 km. (Photo credit: ©USGS).

Fig. 5.96 This 10 km wide delta shows that the forces of the sea are not dominant and delta sedimentation and progradation is not transformed by wave action. West of the Mississippi delta at 29° 30´N and 91° 26´W. (Image credit: © Google Earth 2010).

Fig. 5.97 Deltas sheltered from strong waves and currents show the fluvial regime dominating the marine one. Example from Venezuela at 10° 10´N and 75° 32´W with a width of the image of 9 km. (Image credit: © Google Earth 2010).

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