<P> The Blue Nile has a similar pattern . It contributes 17% of the Nile's waters in the low - water season and 68% during the high - water season . In contrast, the White Nile provides only 10% of the Nile's waters during the high - water season but contributes more than 80% during the low - water period . Thus, before the Aswan High Dam was completed in 1971, the White Nile watered the Egyptian stretch of the river throughout the year, whereas the Blue Nile, carrying seasonal rain from Ethiopia, caused the Nile to overflow its banks and deposit a layer of fertile mud over adjacent fields . The great flood of the main Nile usually occurred in Egypt during August, September, and October, but it sometimes began as early as June at Aswan and often did not completely wane until January . </P> <P> The Nile enters Egypt a few kilometers north of Wadi Halfa, a Sudanese town that was completely rebuilt on high ground when its original site was submerged in the reservoir created by the Aswan High Dam . As a result of the dam's construction, the Nile actually begins its flow into Egypt as Lake Nasser, which extends southwards from the dam for 320 km to the border and for an additional 158 km within Sudan . Lake Nasser's waters fill the area through Lower Nubia (Upper Egypt and northern Sudan) within the narrow canyon between the cliffs of sandstone and granite created by the flow of the river over many centuries . </P> <P> Below Aswan the cultivated floodplain strip widens to as much as twenty km . North of Isna (160 km north of Aswan), the plateau on both sides of the valley rises to as much as 550 m above sea level; at Qina (some 90 km north of Isna) the 300 - m limestone cliffs force the Nile to change course towards the southwest for about 60 km before it turns northwest for about 160 km to Asyut . Northward from Asyut, the escarpments on both sides diminish, and the valley widens to a maximum of 22 km . </P> <P> At Cairo, the Nile spreads out over what was once a broad estuary, subsequently filled by silt deposits to form what is now a fertile, fan - shaped delta some 250 km wide at its seaward extremity and extending about 160 km from north to south . The Nile Delta covers approximately 22,000 km (roughly equivalent in area to that of Massachusetts). According to historical accounts from the first century AD, seven branches of the Nile once ran through the delta . According to later accounts, the Nile had, by around the twelfth century, just six branches . </P>

Why does most of egypt's population live in 3 of the land area