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.[3] 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.[3] wikipedia