Beautypically

The Nile is the longest river on earth. It rises in areas of central and east Africa with high levels of precipitation and flows through the desert regions of Egypt as a foreigner river. The lower Nile Valley is a river oasis that has been used intensively for agriculture for thousands of years.

The strong fluctuations in the water level of the Nile are compensated for by the Aswan Dam, the construction of which, however, also led to serious ecological problems in the Nile valley.

With a length of 6671 km, the Nile is the longest river on earth. Its catchment area covers 2.87 million km², an area eight times the size of Germany (Fig. 1).

The Nile arises from two large tributaries: A source river initially flows into Lake Victoria. He leaves it as Victorianil and first flows through Lake Kioga and then Lake Albert. After leaving Lake Albert, it bears the name Bahr el-Djebel (Mountain Nile) and reaches the wide swampy plains of the Sudd in the south of Sudan. After the Sudd and the confluence of the Sobat from Ethiopia, it finally becomes the White Nile.

The more water-rich tributary is the Blue Nile. It has its source in Ethiopia and flows through Lake Tana in the highlands of Ethiopia. North of Khartoum, the actual Nile emerges after its union with the White Nile.

After the confluence of the Atbara, the Nile flows through the last 2700 km without any further tributaries. As a foreigner river, it crosses the Nubian in Sudan and the Libyan or Arabian desert in Egypt.

Between Khartoum and Aswan the Nile breaks through six mountain ranges made of hard rock. The resulting rapids are known as cataracts.

North of Cairo, it flows into the Mediterranean with two large estuary arms (Rosette and Damiette) in the 24,000 km² Nile Delta.

In its box valley, which is up to 20 km wide and up to 350 m deep, and in the Nile delta, one of the largest river oases on earth is located in the midst of hostile deserts.

The Nile is characterized by pronounced seasonal fluctuations in the water flow. This is related to the distribution of precipitation in its catchment area (Fig. 3).

When the rainy season has started in the headwaters of the White Nile near the equator in the summer months and at the same time extraordinarily rich monsoon rains fall in Ethiopia, then the Nile sill, which lasts until November, begins. At Khartoum, the water level of the Nile will then increase 50-fold from its annual low in April. In addition, the Nile transports enormous amounts of fertile mud during this time, which is supplied to it by the Blue Nile and Atbara from the volcanic regions of Ethiopia. In the low tide period in the first half of the year, the water level then drops alarmingly, especially in the lower reaches.

This alternating rhythm of high and low water determined the life of the people in the river oasis on the lower reaches and in the delta of the Nile for thousands of years.

During the Nile threshold, the fields used to be regularly flooded and covered with Nile mud, which constantly renewed their fertility. At low tide, however, the water level no longer reached the fields. The fields threatened to dry up. However, the Egyptian fellahs (farmers) knew how to outsmart the changing water levels of the Nile with sophisticated irrigation systems and to save the excess water into the low water period in the first half of the year. This made the river oasis a very productive cultivated land.

This led the Greek philosopher HERODOT to say that Egypt was a gift from the Nile.

Since the completion of the Aswan Dam in 1970 and the construction of further dams and large weirs on the lower reaches, the natural rhythm of the water flow of the Nile has been interrupted.

The huge Nasser reservoir, over 500 km long, was created through the high dam. It enables regulation of the Nile floods, year-round irrigation and up to three harvests a year.

This intervention in the natural conditions of the Nile valley also had far-reaching negative consequences for the ecology of the bank landscape. The most serious environmental problems are

• the salinisation of the soils through permanent irrigation under the conditions of high evaporation in the hot desert climate,
• the decline in soil fertility due to lack of fertilization from deposited Nile mud.