Though it moves just a tiny fraction of the water carried by the Amazon, Congo, or Niger rivers, the Nile is the world’s longest river. Its main tributaries—the White Nile and the Blue Nile—meet in Khartoum, Sudan, a rain-poor city of nearly 2 million residents that relies on the Nile for irrigation. Well-watered crops line the river banks, and patchworks of croplands (including center-pivot irrigated fields) dot the city’s outskirts.
The Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite acquired this natural-color image on April 26, 2013, near the end of the region’s dry season. Compared to the White Nile, the Blue Nile is skinny and its highly variable flow is near its lowest point at this time of year. Harsh dry seasons and droughts can periodically dry out the Blue Nile completely.
The White Nile and Blue Nile derive their colors from the sediments they carry. Originating in the Equatorial Lakes region, the White Nile is rich in light gray sediments. As this long river meanders over flat terrain, it loses over half of its water to evaporation.
Shorter than the White Nile, the Blue Nile starts in the highlands of Ethiopia and Eritrea, picking up black sediment en route to Khartoum. The Blue Nile is fed by monsoon rains, and when rains are abundant, the Blue Nile can actually flow backward near its confluence with the White Nile.
Upstream from both rivers, residents rely largely on precipitation for farming. Most of the water extraction along the Nile occurs in Sudan and Egypt, where rainfall is too sparse to support crops. More than 120 millio people rely upon Nile waters for irrigation and other uses.
NASA Earth Observatory image by Jesse Allen and Robert Simmon, using EO-1 ALI data from the NASA EO-1 team. Caption by Michon Scott.
The main tributaries of the world’s longest river meet in Khartoum, Sudan.
Sudan’s capital city, Khartoum, translates as “Elephant’s Trunk.” The name describes the shape of the Nile where the Blue and the White Nile Rivers unite to form the single Nile River that flows northward into Egypt. This image shows the rivers near the end of the dry season. The White Nile (western branch) runs through Sudan from Uganda. It originates in equatorial regions, where rainfall occurs throughout the year, and as a result it runs at a nearly constant rate throughout the year. The Blue Nile, nearly dry this time of year, flows out of the Ethiopian highlands, where rainfall is more seasonal. It swells in the late summer and early fall with rains from the summer monsoons. The flow at these times can be so great that the volume is too much for the river’s channel, causing the Nile to flow backward at the junction.