The Blue Nile is so-called because floods during the summer monsoon erode a vast amount of fertile soil from the Ethiopian Highlands and carry it downstream as silt, turning the water dark brown or almost black.
The distance of the river from its source to its confluence has been variously reported as being between 1,460 kilometres (910 mi) and 1,600 kilometres (990 mi). This uncertainty over the length might partially result from the fact that the river flows through a series of virtually impenetrable gorges cut in the Ethiopian Highlands to a depth of some 1,500 metres (4,900 ft)—a depth comparable to that of the Grand Canyon of the Colorado River in the United States.
According to materials published by the Central Statistical Agency, the Blue Nile has a total length of 1,450 kilometres (900 mi), of which 800 kilometres (500 mi) are inside Ethiopia. The Blue Nile flows generally south from Lake Tana and then west across Ethiopia and northwest into Sudan. Within 30 km (19 mi) of its source at Lake Tana, the river enters a canyon about 400 km (250 mi) long. This gorge is a tremendous obstacle for travel and communication from the north half of Ethiopia to the southern half. The canyon was first referred to as the "Grand Canyon" by the British team that accomplished the first descent of the river from Lake Tana to near the end of the canyon in 1968. Subsequent river rafting parties also called this the "Grand Canyon of the Nile". Commercial rafting trips are offered on the length or portions of this canyon "Blue Nile Raft Expeditions".. The power of the Blue Nile may best be appreciated at the Blue Nile Falls, which are 45 metres (148 ft) high, located about 40 kilometres (25 mi) downstream of Lake Tana.
Although there are several feeder streams that flow into Lake Tana, the sacred source of the river is generally considered to be a small spring at Gish Abay, situated at an altitude of approximately 2,744 metres (9,003 ft). This stream, known as the Gilgel Abay (Lesser Abay River), flows north into Lake Tana. Other affluents of this lake include, in clockwise order from Gorgora, the Magech River, the Northern Gumara, the Reb River, the southern Gumara River, and the Kilte. Lake Tana's outflow then flows some 30 kilometres (19 mi) before plunging over the Blue Nile Falls. The river then loops across northwest Ethiopia through a series of deep valleys and canyons into Sudan, by which point it is only known as the Blue Nile.
There are numerous tributaries of the Abay between Lake Tana and the Sudanese border. Those on its left bank, in downstream order, include the Wanqa River, the Bashilo River, the Walaqa River, the Wanchet River, the Jamma River, the Muger River, the Guder River, the Agwel River, the Nedi River, the Didessa River and the Dabus River. Those on the right side, also in downstream order, include the Handassa, Tul, Abaya, Sade, Tammi, Cha, Shita, Suha, Muga, Gulla, Temcha, Bachat, Katlan, Jiba, Chamoga, Weter and the Beles.
After flowing past Er Roseires inside Sudan, and receiving the Dinder on its right bank at Dinder, the Blue Nile joins the White Nile at Khartoum and, as the Nile, flows through Egypt to the Mediterranean Sea at Alexandria.
The flow of the Blue Nile reaches maximum volume in the rainy season (from June to September), when it supplies 70-80% of the water of the Nile proper. The Blue Nile was a major source of the flooding of the Nile that contributed to the fertility of the Nile Valley and the consequent rise of Ancient Egypt and Egyptian mythology. With the completion in 1970 of the Aswan Dam in Egypt, the Nile floods ended for lower Egypt.