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Causes of the Nile’s regular flooding

The Nile River has the characteristics of regular flooding. In northern Sudan, the water usually starts to rise in May and reaches the highest water level in August. After that, the water level gradually decreases, and the water level is low from January to May. Although flooding occurs regularly, the amount of water and the timing of high tides vary widely.

The cause of this phenomenon is the Blue Nile and Atbara Rivers, which are fed by seasonal torrential rains on the Ethiopian highlands. More than 80% of the water of the Nile River is provided by the Ethiopian plateau, and the rest of the water comes from the East African plateau lake. When the flood comes, it will submerge the farmland on both sides of the bank, and when the flood recedes, it will leave a thick layer of silt to form fertile soil.

Four to five thousand years ago, the Egyptians knew how to control the law of floods and use the fertile land on both sides of the river. For a long time, the Nile River Valley has been filled with continuous cotton fields and fragrant rice flowers. Between the Sahara Desert and the Arabian Desert, the meandering Nile River is like a green outline, full of infinite vitality. The White Nile originates in the rainy area of ​​the equator, and its water volume is abundant and stable.

However, after flowing out of the plateau and entering the basin, due to the extremely flat terrain, the water flow is extremely slow, and the plants that grow in the water also delay the progress of the water flow. Under the sunlight in the dry low-latitude area, it evaporates strongly, thus losing a huge amount of water, which can flow to downstream. There is very little water.

The annual average flow of the White Nile at its confluence with the Blue Nile is 890 cubic meters per second, about half that of the Blue Nile. The lower reaches of the Nile River mainly come from the Sobat River, Blue Nile River and Atbara River, which originate from the Ethiopian Plateau, of which the Blue Nile River is the most important. The Sobat River is a tributary of the White Nile. It began to swell in May, and the highest water level appeared in November. At this time, the water level of the Sobat River was higher than that of the White Nile. of evaporation.

The Blue Nile River originates from Lake Tana on the Ethiopian plateau, and its upper reaches are located in tropical mountainous rainy areas with abundant water sources. Due to the strong and distinct seasonality of precipitation, the river flow varies greatly during the year. The amount of water in spring is limited, and the water starts to rise in June, and then rises rapidly and continuously, reaching its peak in early September.

During this period, it will also cause the White Nile to form an influx. From November to December, the water level drops, and after that is the dry season. The minimum flow in the dry season is less than 100 cubic meters per second, which is about 1/60 of the maximum flow in the flood period.

The Atbara River also originates from the Ethiopian plateau. Due to its location in the north, the rainfall is more concentrated, and its drainage area is small, so the flow changes more. In winter, the flow is cut off, and the riverbed becomes a series of small lakes. A long stretch of the Nile flows through the desert, where the water is lost rather than replenished. Since the upper source of the river is a tropical rainy area, there is a huge flow.

Although a large amount of runoff is lost due to evaporation and seepage along the desert, the Nile River can still maintain a long-term flowing river. A river like the Nile that is not formed by the convergence of local runoff, but simply flows through it, is called the Ke River.

The local climatic conditions have no positive effect on the formation of these “Ke Rivers”, only negative effects. Floods from the main stream of the Nile reach Khartoum (the capital of Sudan) in June and reach their highest level in September. The largest flood peak occurred in Cairo in October. Of the total water volume of the Nile, 60% comes from the Blue Nile, 32% comes from the White Nile, and the remaining 8% comes from the Atbara River. However, there are great changes in flood and dry seasons.

During the flood period, the Blue Nile accounted for 68%, the White Nile accounted for 10%, and the Atbara River accounted for 22% of the water volume of the Nile River. At this time, the Atbara River stopped flowing and had no runoff. The size and change of the proportions of the above-mentioned rivers in the water volume of the Nile are closely related to the amount of precipitation in each river basin and the characteristics of seasonal distribution.