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http://earthobservatory.nasa.gov/IOTD/view.php?id=81186 wrote:

[b][color=#0000FF][size=150]Two Niles Meet in Khartoum, Sudan[/size][/color][/b]

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Two Niles Meet
Earth Observatory, May 19, 2013

<<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 million people rely upon Nile waters for irrigation and other uses.>>

The Sothic cycle or Canicular period is a period of 1,461 ancient Egyptian years (of 365 days each) or 1,460 Julian years (averaging 365.25 days each). During a Sothic cycle, the 365-day year loses enough time that the start of the year once again coincides with the heliacal rising of the star Sirius (the Latinized name for Greek Σείριος, a star called Sopdet by the Egyptians, in Greek transcribed as Sothis; a single year between heliacal risings of Sothis is a Sothic year). This rising occurred within a month or so of the beginning of the Nile flood, and was a matter of primary importance to this agricultural society. It is believed that Ancient Egyptians followed both a 365-day civil calendar and a lunar religious calendar.

• Meteoritics and Planetary Science (online 20 May 2013) DOI: 10.1111/maps.12120