Fayum Depression

The Faiyum Oasis (Arabic: واحة الفيوم‎‎ Waḥet El Fayyum) is a depression or basin in the desert immediately to the west of the Nile south of Cairo. The extent of the basin area is estimated at between 1,270 km² (490 mi²) and 1700 km² (656 mi²). The basin floor comprises fields watered by a channel of the Nile, the Bahr Yussef, as it drains into a desert depression to the west of the Nile Valley. The Bahr Yussef veers west through a narrow neck of land north of Ihnasya, between the archaeological sites of El Lahun and Gurob near Hawara; it then branches out, providing rich agricultural land in the Faiyum basin, draining into the large saltwater Lake Moeris (Birket Qarun). The lake was freshwater in prehistory but is today a saltwater lake. It is a source for tilapia and other fish for the local area.

Differing from typical oases, whose fertility depends on water obtained from springs, the cultivated land in the Faiyum is formed of Nile mud brought down by the Bahr Yussef, 24 km (15 miles) in length. Between the beginning of Bahr Yussef at El Lahun to its end at the city of Faiyum, several canals branch off to irrigate the Faiyum Governorate. The drainage water flows into Lake Moeris.

When the Mediterranean Sea was a hot dry hollow near the end of the Messinian Salinity Crisis in the late Miocene, Faiyum was a dry hollow, and the Nile flowed past it at the bottom of a canyon (which was 8000 feet deep or more where Cairo is today). After the Mediterranean reflooded at the end of the Miocene, the Nile canyon became an arm of the sea reaching inland further than Aswan. Over geological time that sea arm gradually filled with silt and became the Nile valley.

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