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When a river enters a lake it is called a lacustrine delta. Deposition is enhanced if the water is saline: since salty water causes small clay particles to flocculate (the meeting of fresh and salt water produces an electric charge which causes clay particles to coagulate and to settle on the seabed).

Conditions favouring deltaic accumulation are:

1. A river with a high sediment load, such as the Nile or the Mississippi.
2. Usually a large river. This condition is necessary for marine deltas otherwise the action of the sea might disperse the sediment.
3. Reasonably shallow water offshore. Very deep water inhibits delta building. For example, the Congo, which virtually debouches into a submarine canyon, has no delta.
4. A coast on which the wave energy is low, such as the Mediterranean or the Gulf of Mexico, though here again how low will depend on other factors such as sediment supply and tidal range.

There are many delta types, but the three ‘classics’ are:

Arcuate

Fan-shaped, having a rounded, convex outer margin; these are found in the areas where regular longshore drift of other currents keep the seaward edge of the delta trimmed and relatively smooth in shape.

Cuspate

Pointed like a tooth or cusp shaped by regular but opposing, gentle water movement.

Bird’s Foot

Where the river brings down enormous amounts of fine silt, deposition can occur in a still sea area, along the edges of the distributaries for a vast distance offshore.

Millions of Egyptians could be forced permanently from their homes, the country's ability to feed itself devastated. That's what likely awaits this already impoverished and overpopulated nation by the end of the century, if predictions about climate change hold true.

The World Bank describes Egypt as particularly vulnerable to the effects of global warming, saying it faces potentially ''catastrophic'' consequences.

''The situation is serious and requires immediate attention. Any delay would mean extra losses,'' said Mohamed el-Raey, an environmental scientist at Alexandria University.

A big reason is the vulnerability of Egypt's breadbasket the Nile Delta, a fan-shaped area of rich, arable land where the Nile River spreads out and drains into the Mediterranean Sea.

Largely Desert

Although the Delta makes up only 2.5 per cent of Egypt's land mass, it is home to more than a third of this largely desert country's 80 million people.

The Delta was already in danger, threatened by the side effects of southern Egypt's Aswan Dam. Though the dam, completed in 1970, generates much-needed electricity and controls Nile River flooding, it also keeps nutrient sediment from replenishing the eroding Delta.

Add climate change to the mix, and the Delta faces new uncertainties that could have a potentially more devastating effect on Egypt. Scientists generally predict that the Mediterranean, and the world's other seas, will rise between one foot (30 centimeters) and 3.3 feet (one meter) by the end of the century, flooding coastal areas along the Delta. Already, the Mediterranean has been creeping upward about .08 inches annually for the last decade, flooding parts of Egypt's shoreline.

Wipe sandy beaches

By 2100, the rising waters could wipe out the sandy beaches that attract thousands of tourists. Also at risk would be the buried treasures archaeologists are still uncovering in ancient Alexandria, once the second most important city in the Roman Empire. But those losses would pale to the impact of the worst-case scenario that some scientists are predicting global warming unexpectedly and rapidly breaking up the Greenland and West Antarctic ice sheets.

A rise of 3.3 feet (one meter) would flood a quarter of the Delta, forcing about 10.5 per cent of Egypt's population from their homes, according to the World Bank.

The impact would be all the more staggering if Egypt's population, as expected, doubles to about 160 million by the middle of the century.

The Delta is already densely packed with about 4,000 people per square mile (2.6 square kilometer). Also hit would be Egypt's food supply. Nearly half of Egypt's crops, including wheat, bananas and rice, are grown in the Delta. Areas not under water would also be affected, with salt water from the Mediterranean contaminating the fresh ground water from the Nile River used for irrigation.

Fragile Ecosystem

But the unique and fragile ecosystem of the Delta makes the job of protecting it much greater and human activity has already made the task harder.

For thousands of years, annual Nile floods deposited mud, sand and minerals that replenished the Delta and prevented erosion. But for the past three decades, the Aswan Dam has curbed the sediment from resettling in the Delta and allowed erosion to flourish.

In Egypt, as in much of Africa, global warming is rarely discussed. But the government in Cairo is beginning to confront the problem. In Alexandria, authorities are spending $300 million to build concrete sea walls to protect the beaches along the Mediterranean, Frihy said.

Sand is being dumped in some areas to replenish dwindling beaches. Similar walls are going up in other parts of the coast including Rashid, where archaeologists in 1799 discovered the Rosetta Stone that unlocked the secrets of ancient Egyptian writing. The government is also preparing a ''national strategy study'' on ways to adapt to climate change, said Maged George, Egypt's minister of environmental affairs.

Adapted from Source

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