Attachment
13
2.3Hydrology
2.3.1Surface water
The major water towers in Athi catchment include Aberdares, Ngong Hills, Kyanzavi Hill,Kanzalu, Iveti, Kilungu, Mbooni Hills, Chyulu, Taita Hills and Shimba Hills. Athi River is thesecond longest river (540km) in Kenya and its tributaries constitute the main rivers of thecatchment. Perennial rivers include Mbagathi, Ngong, Ruiru, Thiririka, Nairobi, Mathare,Ndarugu, Kiboko, Kibwezi, Lumi and Tsavo rivers. Emphemeral tributaries include the Thwake,Kaiti, Muooni, Makindu and Mtito Andei rivers. Lumi River in Taveta fed by the Njoro KubwaSpring forms a transboundary water resource between Kenya and Tanzania. In the Coastalzone besides Sabaki/Athi, Mkurumudzi, Ramisi, Mwache and Umba are significant thoughsmall rivers. Significant lakes in the catchment include Jipe, Challa and Amboseli while springsinclude Kikuyu, Mzima, Njoro kubwa, Marere, No-turesh, Olpsare, Umani and Kibogo Springs.
2.3.2Groundwater
The Athi catchment region is characterised by varying geology and hydro geological conditionswhich leads to different aquifers. The upper zone is predominantly volcanic with relativelygood water yielding aquifers. The middle zone has localized aquifers in the fractured andweathered zones of the Basement system, with alluvial aquifers being locally important.Chyulu Hills host the aquifer that supplies the Mzima Springs, as well as the spring units thatflow from the eastern side of the range. Volcanic aquifers on the northern flank of Kilimanjaroare sources of a number of springs that are of water supply, irrigation and tourism values(Nolturesh, Kimana, Entonet respectively). The Coastal zone is susceptible to sea waterintrusion and pollution from domestic and industrial point sources. The coastal hinterlandaquifers are generally poor and often brackish.
Table showing groundwater availability in Athi Catchment
Groundwater availability ( m
3
/year)010,00020,00030,00040,00050,00060,00070,00080,00090,000
Ruiru Nairobi
2.3Hydrology
2.3.1Surface water
The major water towers in Athi catchment include Aberdares, Ngong Hills, Kyanzavi Hill,Kanzalu, Iveti, Kilungu, Mbooni Hills, Chyulu, Taita Hills and Shimba Hills. Athi River is thesecond longest river (540km) in Kenya and its tributaries constitute the main rivers of thecatchment. Perennial rivers include Mbagathi, Ngong, Ruiru, Thiririka, Nairobi, Mathare,Ndarugu, Kiboko, Kibwezi, Lumi and Tsavo rivers. Emphemeral tributaries include the Thwake,Kaiti, Muooni, Makindu and Mtito Andei rivers. Lumi River in Taveta fed by the Njoro KubwaSpring forms a transboundary water resource between Kenya and Tanzania. In the Coastalzone besides Sabaki/Athi, Mkurumudzi, Ramisi, Mwache and Umba are significant thoughsmall rivers. Significant lakes in the catchment include Jipe, Challa and Amboseli while springsinclude Kikuyu, Mzima, Njoro kubwa, Marere, No-turesh, Olpsare, Umani and Kibogo Springs.
2.3.2Groundwater
The Athi catchment region is characterised by varying geology and hydro geological conditionswhich leads to different aquifers. The upper zone is predominantly volcanic with relativelygood water yielding aquifers. The middle zone has localized aquifers in the fractured andweathered zones of the Basement system, with alluvial aquifers being locally important.Chyulu Hills host the aquifer that supplies the Mzima Springs, as well as the spring units thatflow from the eastern side of the range. Volcanic aquifers on the northern flank of Kilimanjaroare sources of a number of springs that are of water supply, irrigation and tourism values(Nolturesh, Kimana, Entonet respectively). The Coastal zone is susceptible to sea waterintrusion and pollution from domestic and industrial point sources. The coastal hinterlandaquifers are generally poor and often brackish.
Table showing groundwater availability in Athi Catchment
Groundwater availability ( m
3
/year)010,00020,00030,00040,00050,00060,00070,00080,00090,000
Ruiru Nairobi