The three Great Floods, which occured in the ancient Eyelian kingdom, in the region west of the Zeiphyrian Forests, near today's Chylikis when the Thaehelvil river flooded the lands. The Ylfferhim and Quaelhoirhim record the event in the Már'géf, a ballad that records the fate of the two tribes after the First and Second Great Floods until the death of the Ylfferhim Leias Neve'Nmal.

The First Flood (804 b.S.). From 806 to 804 b.S. extensive deforestation of large tracts of the Zeiphyrian Forest took place as the area came under repeated attack in the First Sarvonian War, was cleared by human forces to allow easy access, both on foot and by boat, to the fort of Elving, and to provide materials for fortifications and naval technology on both sides. The worst affected area was that around the stretch between the present border of the Zeiphyr and the Thaehelvil river. On the first day of the Avénni'modía (Month of the Awakening Earth) 804 b.S. the rains came. Heavy rainfall saturated the forest floors. Where trees once would have intercepted the water, rain flowed straight into the Thaevelvil River. On the third day the river burst its banks and flooded a vast swaith of the land either side of the river, its immense power sweeping away everything in its path. The rain continued for three weeks, and the flood took a further two to subside.

This first flood cleared vast tracts of the remaining forest around the Thaevelvil river, widened its course and its flood plain and created the start of the delta with the Aetryam Sea, and created the Aieryn river that today flows through the forest.

Deforestation led to wide spread flooding of the Aieryn, widening the river, depositing fertile alluvium; which made encouraged further clearing of the area - thus creating the huge delta that meets the Aetryam Sea. The loss of life was too high to count, though the elves record that several military encampments were lost to the flood on both sides. Though the humans camped further to the north and east suffered far fewer casualties. The flood allowed the humans to make significant advance on the elves.

The Second Great Flood (796 b.S.). The elves often refer to this as "The Severing". The second flood occured in much the same way but affected a slightly lesser area, but it cleared the last remaining fragments of forest around the river banks, cleaving the Quallian from the main branch of the forest. It also widened the delta and deposited alluvial soils along its banks, making the whole area incredibly fertile.

The Third Great Flood (701 b.S.). In the leaf fall of 701 b.S. a week of almost continuous rain during the ninth month of the year, along with a channelisation and damming of the Aieryn River upstream close to Vezash (designed to provide irrigation of the fertile farmland) caused the Aieryn to flood. The worst affected area was around Yln'faín'tír (the city was built in the middle of a natural lake), where the lake acted like a bottleneck. The disturbance of the lake's normally very calm waters caused considerable ecological disturbance and for two years the fish stocks on which many of the Ylfferhim depended upon were severly depleted causing famine in the western area of the forest. The death of the fish also attracted Mercarto flies, which normally live much further south. The flies live on the blood of livestock and harbour a virus. They infected the Ylfferhim's small herd of sheep and cattle and many wild deer. While harmless to their four legged hosts, when the elves consumed the meat of the game or livestock they contract a deadly disorder the elves call Mór'cárlth. Sporadic outburst continued until about 628 b.S.