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Résumé :
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Borullus, the most centrally situated of the Nile Delta lakes, probably evolved around the eighth century AD from a preexisting salt marsh by fluviatile deposition of sand dunes north of the lake and subsidence of the preexisting tidal swamp behind this barrier. It was flooded yearly (September-December) by the Sebennytic branch of the Nile, and evacuated water through an exit, Bughaz. At low river levels, this process reversed and Bughaz functioned as a marine inlet. Because of this switch, its fauna and flora contained a mix of marine, freshwater, and brackish-water species. Around the mid-nineteenth century, damming of the Nile began, culminating with the high Aswan Dam (1964) that brought the yearly flood fully under control. As a result, a steady flow of Nile water, used for irrigated delta agriculture, began to drain to the lake and became a constant evacuator to the Mediterranean. It turned almost fresh, and its fishery, formerly marine and mullet-based, became cichlid-catfish based. However, rice and other new delta crops caused huge amounts of nutrients to wash down the drains, and currently the lake is eutrophied and only resists hyper-trophication because of the low residence time of its water. Finally, the damming of the Nile terminated the influx to the delta of a yearly sediment layer, but subsidence and coastal erosion continue and are now consuming the sand bar that separates the lake from the sea.
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