Short answers, pulled from the story.
The Nile is 7,088 km long, making it the longest river in the world. That distance was measured in 2009 along the centerline of the river using satellite imagery, running from a source spring in Rwanda to the Mediterranean Sea.
The source of the Nile is a tributary of the Rukarara River in Rwanda's Nyungwe National Park, at an elevation of 2,539 m. The river flows north for 7,088 km and empties into the Mediterranean Sea near Alexandria, where it has formed a large delta.
The White Nile is longer and is considered the headwaters, beginning near Lake Victoria and flowing through Uganda and South Sudan. The Blue Nile begins near Lake Tana in Ethiopia and contributes over twice the volume of the White Nile, supplying 54% of the main Nile's annual flow. The two rivers meet at Khartoum.
The Nile was the foundation of ancient Egyptian civilization because its annual flooding deposited nutrient-rich silt that supported crops in an otherwise inhospitable desert. The river also drove the development of mathematics, administration, astronomy, and a calendar divided into the three flood-based seasons of Akhet, Peret, and Shemu.
Eleven countries lie wholly or partly within the Nile Basin: Burundi, the Democratic Republic of the Congo, Egypt, Eritrea, Ethiopia, Kenya, Rwanda, South Sudan, Sudan, Tanzania, and Uganda. The basin covers 2,927,843 km, about 10% of the African continent.
The Grand Ethiopian Renaissance Dam on the Blue Nile, announced by Ethiopia in 2011, set off disputes because Egypt feared water shortages and Sudan worried about safety and supply. Numerous negotiations followed, and by 2025 the reservoir was full with most generators producing electricity at a capacity of 5,150 MW.
The Blue Nile's source at Gish Abay was visited by the Jesuit missionary Pedro Paez in the early 17th century. The White Nile proved harder, with John Hanning Speke becoming the first European to see Lake Victoria in 1858, and academics using satellite imagery in 2009 to refine the source to a spring in Rwanda.