Tuesday, December 28, 2010

Day 11 – A brief history of the Nile

The ride on the train was all I had expected to be, sans crime. To a devotee of the novels of Hercule Poirot, I couldn’t help but hope that someone would be bumped off in the middle of the night, and that peering out of my door I would see a woman with a dragon-embroided bath robe walking toward the opposite side of the corridor. Alas, it was not to be, and I had to satisfy myself with a good dinner, a rocking sleep, and a fabulous sunrise over a landscape dotted with palm trees.

I spent the day taking care of the business that brought me to Egypt on the first place: A conference on the development of water projects in arid lands. Most interesting is what I learned about the Nile. The history of the Nile can be described in four stages.

The play starts with the opening of the Red Sea, during the Oligocene. At that time the eastern edge of the African continent was uplifted, as an angry sore around the rifting area. This diverted the flow of water to the west, so there was no “Nile” flowing from south to north at this time. Then something catastrophic happened: Africa nudged itself north against Europe. Two consequences of this nudging were: (1) Deformation and mountain building in Egypt, creating a “fabric” of folds, or ridges and basins, with a general northwest orientation. One of these ridges (anticlines) was to become the el-Fayum depression at a later time. These folds blocked the east-to-west streams and the Nile 1 was formed. The Nile 1 meandered all across Egypt, carrying a goodly amount of water and the sediments eroded from the Red Sea Mountains until 6 million years (Ma) ago, when the second consequence of the collision between Africa and Europe was felt. (2). 6 Ma ago the Strait of Gibraltar closed. Cut off from the inflow of Atlantic water the Mediterranean dried out! Yes, the large volume of the Nile 1 notwithstanding, evaporation in this hot area outpaced the rate of recharge. A thick layer of salt and gypsum accumulated at the bottom of the Mediterranean, which lay 3,000 m below current sea level. From the standpoint of the river, such a gargantuan drop in base level caused the river to aggressively cut down its channel, carving a canyon that rivaled the Grand Canyon in all respects but one; instead of being only 320 km long, like the Grand Canyon, the Nile Canyon was 1,300 km long! Imagine what a rafting trip that would have been :)

5.4 Ma ago, the “dam” at the Strait of Gibraltar broke off, and the Atlantic cascaded into the Mediterranean with a deafening roar. No other waterfall on Earth could have rivaled this one! Now, what happens when sea level suddenly rises? The ocean invades the land canyons, forming estuaries that extend deep into the land. Since the Miocene canyon of the Nile 1 was so long and deep, the estuary was unusually long and deep, and for a couple of million years marine/estuary deposition dominated central Egypt. The river had disappeared in the meantime, but its tributaries kept bringing sediment to the estuary until eventually the Miocene canyon was all filled up with sediment, and the Nile 2 was established.

3.3 to 1.8 Ma ago, the Nile 2 ran over the marine/estuarine deposits, and started carving a not very deep but fairly wide valley. The climate was more humid, and large tributaries in Sudan and Western Egypt contributed significant amounts of water to the river. Then climate changed, quite suddenly as climate is wanton to do, and a very dry period ensued. The dry conditions lasted between 1.8 and 0.8 Ma ago, and they were so severe that the Nile stopped flowing.

The end of the dry period, 800 thousand years (ka) ago, brought the Nile 3 back with a vengeance, and it is largely during this time that the broad valley of the Nile gets its final remodel. This is probably also the time when a channel breaks off the main stem of the river to pour water into the el-Faiyum depression. The origin of this depression, which is now as low as 50 m below sea level is still unclear. Geologically it is an anticline, so it should be high rather than low. Perhaps the folding caused fissuring on the crest of the anticline, formation of caverns, and collapse of an extensive cave system to form the depression. In any case, if you imagine the Nile as a carnation, with the stem being the Nile flowing from south to north, and the delta being the flower itself, el-Faiyum would be the only leaf branching off the stem. All the way up into pharaonic time, in times of flood, the Bahr Yusef flood branch of the Nile has flowed through the Hawara Channel into el-Faiyum, feeding a lake that formed and dried out between big flood events. It was a natural detention basin for the flood waters of the Nile. The ancient Egyptians recognized this, and made sure the channel was kept clear of sediment that could clog the channel and impair the spillway function. Sometime in Ptolemeic times sluice gates were added to the channel, so irrigation into el-Faiyum could be controlled, for the growth of cash crops to be sold to Rome.

Another dry spell starting at 400 ka and ending at 12.5 ka, caused the Nile 3 to flow intermittently (interestingly, between 15 and 11.5 ka is when California has the very wet climate that we geologists refer to as the Pluvial Period of California history.
So at 12,500 years ago the Nile 4 gets established. Much smaller that its predecessor, the Nile 4 is content in flowing over a small portion of the broad alluvial valley, moving around the sediments accumulated by the Nile 3. At Aswan, during construction of the high dam, geologists found over 250 meters of alluvial sediments under the current channel of the river, which go back to the filling of the Miocene canyon by the estuarine and alluvial deposits of Nile 2.

Egypt, and all of the Sahara for that matter, had its own Pluvial Period from about 10,000 to 4,500 years ago. The rain belt of the intertropical convergence region shifted northward just enough to turn the desert into a savannah, and much of Egypt’s groundwater must have been recharged during this time. Then climate started deteriorating, and civilization sought the Nile 4 valley as a cradle to develop. Although the evidence is at best permissive, it seems that the three golden periods of Egyptian civilization may have coincided with times when the flow of the Nile was average or above average, whereas the two intermediate periods when government collapsed may have coincided with periods when the flow of the Nile was well below average for an extended period of time.

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