Lake Naivasha is a shallow freshwater lake in the southern Rift Valley, surrounded by acacia woodlands. The Lake is considered to be of recent geological origin and is ringed by extinct or dormant volcanoes. It is fed by the perennial Malewa and Gilgil Rivers, which drain the Aberdare Mountains of central Kenya. Although the Lake has no visible outlet, its water is fresh; it is thought that a combination of underground seepage and sedimentation of salts keeps the lake fresh, unlike other endorheic lakes in the eastern Rift Valley. Papyrus reeds (Cyperus papyrus) line much of the shoreline, with variable areas of submerged plants such as Potamogeton species, and floating rafts of the exotic water hyacinth Eichhornia crassipes. Lake Naivasha is a highly significant national freshwater resource in an otherwise water-scarce area, supporting a diverse community of waterbirds and many large mammals. In addition to the invaluable freshwater, it also supports large-scale and vital economic activities, mainly flower growing, fishing and geothermal power generation. The ecology of the Lake and its surroundings is fragile, with dynamic ecosystems and a still uncertain water balance in a basin surrounded by intensively irrigated agricultural land and a rapidly growing township.