This site lies on the floor of the Rift Valley, 80 km north-west of Nairobi, and consists of a shallow freshwater lake (15,600 ha) and its fringing Acacia woodland (c.7,000 ha). Lake Naivasha is of recent geological origin, and is ringed by extinct or dormant volcanoes, including Mounts Longonot, Ol Karia and Eburu. Naivasha’s water is supplied by the permanent Malewa and Gilgil rivers, which respectively drain the Aberdare mountains (IBA KE001) and the Rift Valley floor to the north, by the seasonal Karati river (also draining from the Aberdares) and from substantial ground-water seepage. The Malewa covers 1,730 km2 of the 2,800 km2 catchment, and contributes 90% of the surface water entering the lake. Naivasha has no surface outlet. It is thought that a combination of underground outflow and sedimentation of salts keeps the lake fresh, unlike other endorheic lakes in the eastern Rift Valley. Naivasha includes three chemically distinct water bodies. The main lake (c.15,000 ha, maximum depth c.8 m) incorporates a partially submerged crater, the Crescent Island lagoon (maximum depth c.18 m), at its eastern end. The lagoon is largely isolated at low water levels. To the south-east, separated by papyrus Cyperus papyrus swamp and an isthmus of Acacia woodland, is the small (c.550 ha), somewhat alkaline Lake Oloidien. Papyrus fringes the main lake’s shore (with scatterings of other sedges and Typha) and cloaks the inlets of the Gilgil and Malewa rivers. There are large floating, wind-driven rafts of the exotic water-hyacinth Eichhornia crassipes, usually concentrated in the south-west sector. Submerged macrophytes (including Potamogeton spp. and Naja pectinata) sometimes occur in large beds, mainly in the shallow eastern part, but these vary greatly in extent. The shores of Crescent Island lagoon are steep and rocky or flat and muddy, while Oloidien has an open, grassy shoreline, with no emergent or floating macrophytes. The lake’s levels fluctuate enormously, and Naivasha has been dry within historic times. The surrounding riparian land is almost all privately owned, much of it now used for intensive horticulture and floriculture using water from the lake. A belt of tall Acacia xanthophloea woodland fringes the lake and extends along the rivers to the north, though portions have been cleared for farming; further from the water this gives way to dry open grassland, Tarchonanthus camphoratus scrub and (on rocky hillsides) Euphorbia forest. Naivasha is the second site listed by Kenya as a Wetland of International Importance under the Ramsar Convention.
See Box and Table 2 for key species. The woodland north of the lake and around Lake Oloidien provides habitat for Prionops poliolophus, which has been recorded here regularly and is known to nest. Acrocephalus griseldis is a winter visitor and passage migrant, the exact status of which is unknown. The lake itself supports a diverse waterbird community, with more than 80 species regularly recorded during censuses. Mean numbers during 1991–2001 were 19,600 waterbirds. Depending on water levels, it can be a significant site for Fulica cristata (mean 5,050 during 1991–2001), Platalea alba (mean 138 during 1991–2001) and Tachybaptus ruficollis (mean 650 during 1991–2001). Many species of duck and Palearctic waders also occur in numbers; Palearctic duck are especially abundant in November and February. Phoenicopterus minor occurs in small numbers at times, mainly on Oloidien. The lake is known for its high density of Haliaeetus vocifer, which nest in the surrounding Acacia woodland. Regionally threatened species include Podiceps cristatus (most recent Kenyan records are from Oloidien, with seven birds seen in January 1996); Oxyura maccoa (regular on Oloidien, with 170 in January 1994 and January 1997); Anhinga rufa (one recorded on Oloidien in January 1997); Casmerodius albus (regular at Naivasha, which is an important feeding site; 73 counted in January 1997); Ephippiorhynchus senegalensis (2–3 birds usually present); Thalassornis leuconotus (occasional; 12 counted on Oloidien in January 1994); Porzana pusilla (status uncertain); and Rynchops flavirostris (irregular visitor). Since 1995 a large nesting colony of Phalacrocorax carbo has established itself in the fringing Acacia woodland at Lake Oloidien.
Non-bird biodiversity: The lake supports a large and expanding population of Hippopotamus amphibius (c.300 individuals at present). The snake Bitis worthingtonii, endemic to the central Rift Valley above 1,500 m, is recorded from Naivasha.
Pressure/threats to key biodiversity
The conservation issues surrounding Naivasha are complex, with the effects of human pressures superimposed on a naturally variable system. The lake has seen dramatic ecological changes through the effects of introduced species. In the 1960s and 1970s, Mycocaster coypu (now probably extinct) and the crayfish Procambarus clarkii ate their way through the lake’s submerged plants and extensive beds of water-lilies (Nymphaea sp., now almost vanished). The floating plant Salvinia molesta, a problem in the 1980s, was largely replaced in the 1990s by the notorious water-hyacinth Eichhornia crassipes. Eichhornia appears to provide excellent shelter for young and adult crayfish, contributing to a crustacean population boom that is preventing any recovery of the submerged macrophytes. This in turn has negative effects on several bird species, notably the Fulica cristata, numbers of which have declined dramatically. All the lake’s five fish species are introduced. Two tilapias (Oreochromis leucostictus and Tilapia zillii) and the predatory Micropterus salmoides support a once-productive fishery. Much fishing goes on illegally, often with small-mesh gill-nets that catch undersized fish, and over-exploitation has caused fish populations to slump. Gill-netting itself is a cause of high mortality in diving birds such as Podiceps cristatus and Anhinga rufa. These two regionally threatened species, once common on the lake, have nearly vanished; most recent records are from Lake Oloidien, which is not fished. Increasing human pressures are the major threat to Naivasha. Since the late 1980s the area has seen an extraordinary explosion of horticulture and floriculture for the European export market. The climate and soils, and the ready supply of irrigation water, are ideal for intensive production of cut flowers and crops such as green beans. Large areas of woodland and fringing swamp have been cleared, with cultivation sometimes extending right down to the lake edges. Irrigated agriculture extracts large quantities of water from the lake, as does the nearby geothermal power plant at Ol Karia (now being expanded in a second phase). Water extraction may now exceed replenishment, although no adequate water budget is available. Intensified cultivation and the removal of fringing swamps has increased the amount of sediment and nutrients (along with potentially hazardous agricultural chemicals) reaching the lake. Turbidity and algal biomass increased rapidly throughout the 1980s. Waterbird populations, especially of Fulica cristata, kingfishers and Haliaeetus vocifer, showed significant declines from 1991–1997, but have increased again following a rise in lake levels during 1998. With spectacular scenery, a fine climate, tranquil surroundings and easy access, Naivasha is already an important site for local and international tourism. Visitors come for bird watching, hippo viewing and watersports, and to visit the nearby National Parks of Hell’s Gate and Longonot. Administratively the lake is entirely within Naivasha town, but almost all the land around it is privately owned. The Ministry of Water Development and Department of Fisheries are in charge of controlling water and fish exploitation, respectively, but it is the riparian landowners that can contribute most to management efforts. The Lake Naivasha Riparian Owners’ Association was instrumental in having Naivasha declared a Ramsar Site in 1995, and in drawing up a comprehensive management plan that was accepted by Government in 1997 and is presently being implemented. The woodland north of the lake and around Lake Oloidien has generally been well conserved by landowners. Elsewhere, large tracts have been cut to make way for greenhouses. This may be having an impact on the populations and movements of Prionops poliolophus, which are poorly understood, and further work on this species is required.