Lake Tana is the source of the Blue Nile

Blue Nile

The Blue Nile is a river originating at Lake Tana in Ethiopia. With the White Nile, the river is one of the two major tributaries of the Nile...

 and is the largest lake

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

 in Ethiopia

Ethiopia

Ethiopia , officially known as the Federal Democratic Republic of Ethiopia, is a country located in the Horn of Africa. It is the second-most populous nation in Africa, with over 82 million inhabitants, and the tenth-largest by area, occupying 1,100,000 km2...

. Located in Amhara Region

Amhara Region

Amhara is one of the nine ethnic divisions of Ethiopia, containing the homeland of the Amhara people. Previously known as Region 3, its capital is Bahir Dar....

 in the north-western Ethiopian highlands

Ethiopian Highlands

The Ethiopian Highlands are a rugged mass of mountains in Ethiopia, Eritrea , and northern Somalia in the Horn of Africa...

 the lake is approximately 84 kilometers long and 66 kilometers wide, with a maximum depth of 15 meters, and an elevation of 1,840 meters . Lake Tana is fed by the Lesser Abay

Lesser Abay

Lesser Abay is a river of central Ethiopia. Rising in the mountains of Gojjam, it flows northward to empty into Lake Tana at . Tributaries of the Lesser Abay include the Ashar, Jamma, Kelti and the Koger....

, Reb

Reb River

Reb River is a river of north-central Ethiopia which empties into Lake Tana at . The river originates on the slopes of Mount Guna, and flows west through Kemekem woreda. It has no significant tributaries.R.E...

 and Gumara River

Gumara River

Gumara River is a river of northern-western Ethiopia and empties into Lake Tana at from the east. Hot springs on its banks at Wanzagay, which were popular in medicinal hot baths in the late 18th and early 19th centuries, are mentioned by the missionary Henry Stern.The river is an important...

s and its surface area ranges from 3,000 to 3,500 km² depending on season and rainfall. The lake level has been regulated since the construction of the control weir

Weir

A weir is a small overflow dam used to alter the flow characteristics of a river or stream. In most cases weirs take the form of a barrier across the river that causes water to pool behind the structure , but allows water to flow over the top...

 where the lake discharges into the Blue Nile

Blue Nile

The Blue Nile is a river originating at Lake Tana in Ethiopia. With the White Nile, the river is one of the two major tributaries of the Nile...

, which regulates the flow to the Tis Abbai falls and hydro-power station.
The lake has a number of islands, whose numbers vary depending on the level of the lake; it has fallen about 6 feet (1.8 m) in the last 400 years.