The lake lies in a trough below the Ngendelel Escarpment, a sheer wall 600 metres (2,000 ft) high. The lake covers 32 square kilometres (12 sq mi). It is geothermically active on the western shore, with geysers and hot springs. The geologist J.W. Gregory described the lake in 1892 as "the most beautiful view in Africa".
The reserve is in a semi-arid area. The only major river feeding the lake is the Waseges River, which rises on the northern slopes of the Aberdare Range. The Waseges runs through productive agricultural land higher up, through bush and scrub used for grazing, and then through very dry bush before entering the lake at its northern end. The lake is surrounded by grasslands dotted with bushes. There is acacia-ficus woodland to the south, and the north merges into a papyrus swamp.