There was little detailed knowledge, in antiquity, of the hydrology of the Nilethe whims of ancient . As such its periodic rise was attibuted to Gods until the discovery of the role of the tropical regions, specifically the Ethiopian highlands"discovery" of its source. , in its regime, the Ancient Egyptians made early records of the river level with the aid of nilometers scales cut in natural rocks or (gauges formed by graduated in stone walls), some of which still remain.

The Nile swells in the Egyptian summer, the floods rising as a result of the heavy tropical rains in Ethiopia. In the southern Sudan the flood begins in April, but the effect is not felt at Aswan, Egypt, until July. The water then starts to rise and continues to do so throughout August and September, with the maximum occurring in mid-September. At Cairo the maximum is delayed until October. The level of the river then falls rapidly through November and December. From March to May the level of the river is at its lowest. Although the flood is a fairly regular phenomenon, it occasionally varies in volume and date. Before it was possible to regulate the river, years of high or low flood--particularly a sequence of such years--resulted in crop failure, famine, and disease. These are still prevalent in the Nile riparian nations that have yet to develop and utilize this important resource.

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Contributions of Several sources (1)
Following the river from its sources, an estimate can be made of the contribution of the various lakes and tributaries in the Nile flood.

The White Nile (Nech Abay)
Lake Victoria forms the first natural reservoir of the Nile system. Its heavy rainfall is almost balanced by surface evaporation, and the lakes 812 billion cubic feet (23 billion cubic metres) outflow mostly from the rivers draining into it, particularly the Kagera. This water then flows as the Victoria Nile into Lake Kyoga, , and then into Lake Albert. Water lost to evaporation is more than balanced by rainfall over the lake and inflow from other smaller streams, notably the Semliki. The annual outflow from Lake Albert to the Al-Jabal River is about 918 billion cubic feet.

The seasonal (torrential) tributaries of the Al-Jabal supply it with nearly 20 percent of its water. The outflow from the Al-Jabal varies little throughout the year because of the regulatory effect of the large swamps and lagoons of the As-Sudd region. Though half of its water is lost, at this stage ,to seepage and evaporation, the flow from the Sobat River into the main stream just upstream of Malakal nearly makes up for the loss.

The White Nile provides a regular supply of water throughout the year. During April and May, when the main stream is at its lowest level, more than 80 percent of its water comes from the White Nile. The White Nile obtains its water equally from the rainfall on the East African Plateau of the previous summer and the drainage of southwestern Ethiopia through the Sobat( the Baro and the Pibor) that enters the main stream below As-Sudd. The annual flood of the Sobat, due to Ethiopian summer rains, is responsible for variations in the level of the White Nile. Rains swell its upper valley beginning in April causing inundation over the 200 miles of plains through which the river passes, delaying the arrival of the rainwater in its lower reaches until November-December. Relatively small amounts of the mud carried by the Sobat's flood reach the White Nile.

The Blue Nile(T'Kur Abay)
The Blue Nile, plays an overwhelming part in bringing the Nile flood to Egypt. It receives two tributaries in Sudan--the Ar-Rahad and the Ad-Dindar--both of which also originate in Ethiopia. The regime of the Blue Nile is more rapid in the passage of its floodwater into the main stream. The river level begins to rise in June, reaching a maximum level at Khartoum in about the first week in September.

The Atbara River draws its floodwater from the rains on the northern part of the Ethiopian Plateau, as does the Blue Nile. But while the floods of the two streams occur at the same time, the Blue Nile remains perennial, while the Atbara, shrinks to a series of pools in the dry season.

The rise of the Blue Nile causes the first floodwaters to reach central Sudan in May with the maximum occuring in August, after which the level falls again. The rise at Khartoum averages more than 20 feet. When the Blue Nile is in flood it holds back the White Nile water, turning it into an extensive lake and delaying its flow.

The peak of the flood does not enter Lake Nasser until late July or August, when the average daily inflow from the Nile rises to some 25.1 billion cubic feet. Out of this amount the Blue Nile accounts for almost 70 percent, the Atbara more than 20 percent, and the White Nile 10 percent. In early May the inflow drops to its minimum; the total discharge of 1.6 billion cubic feet per day comes mainly from the White Nile and the remainder from the Blue Nile. On the average, about 85 percent of the water in Lake Nasser comes from the Ethiopian Plateau, and the rest is contributed by the East African Lake Plateau system. Lake Nasser has an enormous storage capacity--more than 40 cubic miles (about 168 cubic kilometres)--although the content of the reservoir varies with the extent of the annual flood upstream. Because it is situated in a very hot and dry region, however, Lake Nasser can lose up to 10 percent of its volume to evaporation annually when it is full, decreasing to about one-third that amount when it is at minimum capacity.
(1) Encyclopedia Britannica

Geography of the Abay (Nile) Valley