Short answers, pulled from the story.
The yearly monsoon between May and August brought heavy rainfall to the Ethiopian Highlands, sending runoff into the Blue Nile and the Atbarah River. During this period, the Atbarah and Sobat rivers together contributed up to 90% of the Nile's total flow and most of its transported sediment.
Wafaa El-Nil is a two-week holiday in Egypt that commemorates the annual flooding of the Nile, beginning on the 15th of August each year. In the Coptic Church, the occasion is also marked by casting a martyr's relic into the river in a ceremony known as the Martyr's Finger.
Beginning around 7000 years ago, Egyptians developed basin irrigation: large fields bounded by dams and levees were flooded for about 45 days to saturate the soil and allow silt to settle, then drained before planting. Crops of cereals, flax, and Nile grass were grown within the seasonal window that followed each year.
The annual flooding of the Nile ended in 1970 with the completion of the Aswan High Dam and the filling of Lake Nasser. The reservoir was designed to contain even the highest recorded floodwaters, ending a cycle that had persisted since antiquity.
In the Ancient Egyptian religion, the floodwaters were understood as the tears of the goddess Isis, shed for her murdered husband Osiris. The god Hapi and the pharaoh together were believed to govern the flooding. During inundation festivals, Egyptians planted mud figures of Osiris packed with barley seeds.
Harold Edwin Hurst was a British hydrologist in the Egyptian Public Works who spent years analyzing the Nile's water-level fluctuations. In 1946, he submitted a 'century storage' plan to manage droughts predicted to occur once in 100 years, proposing new reservoirs at Lake Victoria, Lake Albert, and Lake Tana, and the Jonglei Canal to reduce evaporation in the Sudd.