Delta – watercourse between river and sea

Mighty and sluggish, the Nile pours into the Mediterranean. Like all large rivers flowing through a plain, the African stream slows down towards its mouth. From the slow flow, the cargo of eroded debris and sand sinks to the bottom and is deposited. With these deposits, the river builds its own obstacle that it has to flow around. The result is a finely branched branching of sandbanks, scree slopes and river arms that becomes wider and wider towards the mouth. From the air, this widely branched network looks like a triangle. Because of its shape, it is called a delta – after the Greek letter of the same name.

Over time, the river accumulates layer upon layer of sediment. The course of the river buries its own mouth and the delta protrudes further and further into the sea: the river lengthens. This process is particularly visible on the Nile. Its delta begins near Cairo and is now 160 kilometres long and 240 kilometres wide at the coast. And the Nile delta is getting bigger and bigger: shaped like a fan, it is constantly growing further into the Mediterranean.

For a delta to develop, other conditions must be met. The coast must be flat, the tides and the ocean current must be low, because only then will the sediments not be immediately transported away again by the moving seawater. The right conditions prevail, for example, in the lower reaches of the rivers Po or Danube. Both rivers flow into the shallow sea in a delta.

The race of the rivers

The Nile is the longest river on earth. Or is it not? Some researchers doubt this record and claim that the Amazon is longer. But it is not so easy to measure a river precisely.

The previous leader was the African Nile with a length of 6,671 kilometres. It has its source in two headwaters: The Blue Nile from the Ethiopian highlands and the White Nile from the mountains of Rwanda and Burundi. On its way, it flows through Tanzania, Uganda and Sudan and flows into the Mediterranean Sea in Egypt. So long, so good. But some researchers now claim that the Amazon is longer. Until now, South America’s most gigantic river had an official length of 6,437 kilometres. If, however, one were to add its tiniest source river, one would arrive at around 7,000 kilometres. That would make the Amazon clearly longer than its African competitor. And now? Scholars cannot agree. Textbooks and reference books still rank the Nile first among the longest rivers, the Amazon only second. But there is one thing that cannot be disputed about the Amazon: It is by far the most water-rich river on earth: two-thirds of all the river water on earth flows through its bed. At its mouth, 200,000 cubic metres of water flow into the Atlantic Ocean every second – the equivalent of about a million full bathtubs!

The Nilometer

The life of the ancient Egyptians, who lived around 5,000 years ago, was determined by the Nile. It probably didn’t matter to them whether their river was longer than all the others. What was important, however, was that they could cultivate crops on its banks – and that in the middle of Egypt, a land of desert.

Once a year, the Nile overflowed its banks and spread black and fertile mud over meadows and fields – a perfect fertiliser. Because the river and its water level were so crucial to their lives, the ancient Egyptians built Nilometers: They could read from these markers whether the water level was falling or rising. Even taxes were assessed according to the Nilometer: A high water level promised good harvests and meant high taxes. If the Nile had little water, the taxes that the farmers had to pay also decreased.

The cycle of water

The water on earth is always on the move. Huge amounts of it are constantly moving – between sea, air and land – in an eternal cycle in which not a drop is lost.

The motor of the water cycle is the sun: it heats the water of the oceans, lakes and rivers so much that it evaporates. Plants also release water vapour into the atmosphere through tiny openings. The moist air rises upwards, tiny water droplets gather at high altitudes and form clouds. As rain, hail or snow, the water falls back into the sea or onto the earth. If it falls to earth, it seeps into the ground, feeds plants or flows through the ground, via streams and rivers back into the sea. The eternal cycle of evaporation, precipitation and runoff starts all over again.

The water cycle has existed almost as long as the earth has. It ensures that living beings on our planet are supplied with fresh water. And not only that: without the water cycle, the weather as we know it would not even exist.

From trickle to stream – flowing waters

Bubbling groundwater emerges from a spring and flows down the slope as a thin trickle or a small stream: a watercourse has been created. All flowing waters start out small. On their way towards their mouths, they merge with other watercourses and continue to grow until they have become a river or even a broad stream. At its lower end, the watercourse flows into another river, a lake or the sea.

Streams, rivers or creeks – terms that come to our lips fluidly are precisely distinguished from one another by scientists (geographers). They can be classified by their water volume, their length or their width: If the watercourse is less than half a metre wide, it is called a trickle; if it is more than 2 metres wide, it is called a stream. If the watercourse swells to a width of 10 metres, it is a river. And if it gets even wider, the river can be called a stream. The Amazon or the Nile, for example, are called rivers, but the Rhine and the Danube are also streams.

The amount of water in the flowing water increases from the source to the mouth. Nevertheless, it flows slower and slower downhill. This is because the slope down which it flows is steeper at the top than at the bottom. And because the water flows faster at the top and slower downstream, it can carry more sand and debris along the upper course than the lower. Thus, more sand and debris is removed from the upper course of a river, and more is deposited in the lower course.

How does the salt get into the sea?

Anyone who has ever swallowed water while bathing in the sea knows from personal experience: seawater tastes salty. And when the water evaporates, a fine white layer of salt often remains on the skin. This is because seawater consists on average of 3.5 percent salt. For one litre of seawater, that’s 35 grams or about one and a half heaped tablespoons of salt. But how does the salt actually get into the sea?

Many of these salts come from the rocks in the earth’s crust. Rainwater dissolves salts from the rock and takes them with it. It washes them into rivers and groundwater. In this way, salts are washed into the sea. Because relatively little salt is transported, the river water is hardly salty. The concentration only increases in the sea. This is because salts from the ocean floor and from submarine volcanoes are added there. When the seawater evaporates, all these salts remain. This is why salts that have been washed out have been accumulating in the oceans for millions of years.

The salt content is not the same in all seas. The more water evaporates, the more saline the water becomes. The Red Sea contains more salt than the Pacific. And the Dead Sea in the Middle East – actually a lake – is so salty with a salt content of about 30 per cent that you can lie in it without sinking. In contrast, the Baltic Sea is rather low in salt: because of the low temperature, only little water evaporates there. In addition, many rivers flow into the inland sea and feed it with fresh water. That is why the Baltic Sea is much less salty than the Dead Sea.