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 hypertrophication 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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