Hydrology of the Nile

It puzzled the ancients why the amount of water flowing down the Nile in Egypt varied so much over the course of a year, particularly because almost no rain fell there. This puzzlement was compounded for the ancient Romans and Greeks because the Nile's minimum flow was in winter and maximum flood occured during summer, opposite from the variations of the European rivers that they knew. (Click on Aswan on the map below). Today we have hydrographic information that tells us why the Nile is a 'summer river'.

The Nile south of the Great Bend is really two hydraulic regimes: the White Nile system on the one hand and the Blue Nile-Atbara system on the other. As the graph for Malakal shows, the White Nile maintains a constant flow over the year, because its flow is doubly buffered. Seasonal variations are moderated, first because of storage in Central African lakes of Victoria and Albert and second because of evaporative losses in the Sudd, the world's largest freshwater swamp. (Click on Malakal on the map below). The Sudd is especially efficient in reducing annual variations in streamflow. Unusually wet years increase the area of the Sudd which leads to larger evaporative losses than during dry years, when the area of the Sudd is reduced. This even modulates the seasonal variations of the Sobat, which is a seasonal stream that flows west into the Sudd from Ethiopia. The result is that the White Nile issuing from the Sudd - south of Malakal - flows at about the same rate all year long. This steady stream keeps the Nile downstream from Khartoum flowing during the winter months, when the Blue Nile/Atbara system has dried up.

The Blue Nile-Atbara system is a completely different hydraulic regime. It responds to the wet season/dry season variation of the Ethiopian highlands. In the winter, when little rain falls in the highlands, the Atbara and Blue Nile dry up. (Click on Atbara on the map below). But in the summer, when moist winds from the Indian Ocean cool as they climb up the Ethiopian highlands, bringing torrential rains to Ethiopia. Day after day the monsoon drenches the highlands, quickly filling the dry washes and canyons with red rushing water, which pours over the cliffs and down into torrents that ultimately join the Blue Nile or the Atbara. Look at the graphs for Rosieres, on the Blue Nile, and Atbara, at the mouth of the Atbara River. The Blue Nile and Atbara are some of the best examples that we have of seasonal streams. (Click on Roseires on the map below). Notice how little water issues from these streams during winter and early Spring - most of the water during this time of year comes from the White Nile - but see how the Ethiopian streams increase in volume during the summer monsoons! During the summer, the White Nile's contribution is insignificant, a drop in the bucket. The annual flood in Egypt is nothing more or less than the distant echo of the annual monsoon in Ethiopia. In this sense, Egypt is really the gift of the Indian Ocean.

The combined effects of the two systems can be seen on the graph for Dongola, where the summer contribution from the Blue Nile/Atbara system is superimposed on the year-round steady flow from the White Nile. (Click on Dongola on the map below). The same pattern is evident in the pattern for Aswan, only there is less water here due to evaporation of the Nile waters during its leisurely passage through the Sahara Desert. Water is lost due to evaporation - not to mention human usage - so that progressively less water flows in the Nile from Arbara, the Nile's last tributary, all the way to the Mediterranean Sea. (Click on Aswan on the map below).

Click on Location to see Hydrology Graph