The Nile's waters increase in the summer due to the heavy rainfall that occurs in the tropical Ethiopian highlands. In April, flooding begins in southern Sudan. It is July before the floods reach Aswan, Egypt. The river continues to rise until it peaks in mid-September. In Cairo, the floods are delayed until October. After the river has peaked, the levels fall quickly during November and December. The Nile is at its lowest levels between March and May. The annually recurring flooding now able to be controlled by means of the Aswan Dam.
The amount of flood water contributed by the various tributaries and lakes can be calculated by following the river from its multiple sources. Starting with Lake Victoria, 812 billion cubic feet (23 billion cu. m.) is brought in by those rivers that drain into it, most notably the Kagera. Evaporation helps to balance the water that drains into the lake and continues to Lake Albert. The outflow from Lake Albert is around 918 billion cubic feet.
The White Nile's water supply remains steady throughout the year. When the Nile is at its lowest levels throughout April and May, the White Nile actually supplies the river with over 80 percent of its water. The White Nile's water supply remains constant as a result of the two water sources. The first occurs when the rains from the East African Plateau arrive. The second source results from the drainage of southwestern Ethiopia that flows into the Sobat and finally into the Nile.
The Blue Nile is somewhat responsible for the flooding that occurs in Egypt. Two main tributaries originating in Ethiopia come together in The Sudan. Their waters rush into the mainstream and the river levels start to rise in June, reaching their maximum level in Khartoum during early September.
The Atbara River carries the flow from the Ethiopian Plateau along with the Blue Nile. These two flood at the same time. However, the differences between the two help to offset one another somewhat. The Blue Nile's stream is perennial in comparison to the Atbara, which turns into numerous pools during the dry season.
When the waters of the Blue Nile swell, the floodwaters continue their course and finally reach central Sudan in May. By August the rivers level averages more than 20 feet at Khartoum. Fortunately, when the Blue Nile is flooding it holds the White Nile back. A large lake is formed and its flow is delayed. South of Khartoum, the Jabal al-Awliya Dam has been built which increases the pooling effect.
When the flood on the Nile reaches its maximum flood stage, it flows into Lake Nasser with an average daily flow that can rise to 25.1 billion cubic feet. This occurs in the later part of July or August. At this stage the White Nile is responsible for contributing 10%, the Atbara for 20% and the Blue Nile for 70%. Finally, the river's flow decreases in May. It is interesting to note that 85% of Lake Nasser's water originates on the Ethiopian Plateau, the remaining 15% coming from the East African Lake Plateau. Because of its location in a very hot, dry region, the lake loses 10% of its volume annually through evaporation.
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