Lake Tana is located in the northwestern part of Ethiopia with a surface area of 3156 km2, accounting 20% of the 15096 km2 drainage area of the Lake Tana basin. The lake is a natural type which covers 3000 – 3600 km2 area at an elevation of 1800 m and with a maximum depth of 15 m. It is approximately 84 km long, 66 km wide. It is the largest lake in Ethiopia and the third largest in the Nile River basin. It is the main source of the Blue Nile River that is the only surface outflow for the Lake.
The lake receives perennial flow from four major rivers: Gilgel Abay, Rib, Gumera, and Megech contributing 93% of the inflow, and at the outlet starts the Blue Nile River. The mean annual rainfall of the catchment area is about 1280 mm. The annual mean actual evapotranspiration and water yield of the catchment area is estimated to be 773 mm and 392 mm, respectively (Setegn et al., 2009). Records show that Lake Tana had a historical variation in its level believed to stem from hydrologic alterations within its basin due to reduction in dry season flows attributed to human and climate-induced changes.