Green Lake is one of a number of crater lakes near Debre Zeit in East Shewa Zone, 50 km west of the zonal capital, Nazaret. Like all of the volcanic crater lakes in this area, Green Lake is a closed system, being fed directly by rain and by water flowing down from the crater rims. The maximum depth is 32 m, and the water is highly alkaline. The lake is encircled by an almost continuous crater wall with steep and, in places, precipitous slopes rising almost 200 m above the water. Cliffs and scattered trees that grow on these cliffs make up the major habitats surrounding the lake. Drier slopes around the lake support various Acacia spp. where disturbance and grazing are minimal. Severely eroded areas are either bare or carry highly drought-tolerant shrubs, scramblers and succulents, the most conspicuous of which are Carissa edulis, Euphorbia tirucalli, Pterolobium stellatum, Caesalpina spinosa and Opuntia ficus-indica. The water exclusively supports an abundant population of the blue-green alga Spirulina platensis, as a result of which the water appears green (hence the name of the lake). The dense algal growth in the lake produces a huge amount of oxygen during the day, but at night photosynthesis ceases, the algae use up the oxygen, and the lake becomes almost completely anaerobic by daybreak. This phenomenon partly explains the absence of fish from the lake.