Athi Basin Area (ABA)

Athi River passing through the Yatta Platteau (Picture by Mathew Mutie; The Treasure Blog)
Athi River passing through the Yatta Platteau (Picture by Mathew Mutie; The Treasure Blog)


Athi Catchment is bounded by Latitudes 1º to 4.5º South and Longitudes 37º to 40º East. It borders Tana Catchment Area (TCA) in the North, Indian Ocean in the East, Tanzania in the South, and Rift Valley Catchment Area (RVCA) in the West.  The Aberdare Range, one of the major water towers, lies in the northern edge of the area.  Total area of ACA is 58,639 Km2, corresponding to 10.2% of the country’s total area.

Administratively, the drainage basin covers: Nairobi, Makueni, Taita Taveta, Kwale and Mombasa Counties, part of Kiambu, Machakos, Kajiado, Kilifi, Kitui and Nyandarua Counties.

Major cities and towns found in ACA are Nairobi (the Capital City), Mombasa (the second largest), Kiambu, Kajiado, Machakos, Malindi, and Kilifi.  Other upcoming cities include Konza, Machakos, Tatu, and Diani Resort.

Location map of the Basin Area
Location map of the Basin Area

Topography and land cover

 Athi Catchment Area has varied topographical characteristics, from the highland in the Aberdare Ranges of around 2,600M above mean sea level (amsl) to the coastal area at the sea level. It is divided into three zones:

  • Upper zone of 2,600-1,500M (amsl) which covers the Aberdares, Kikuyu Escarpment and Ngong Hills. This zone is predominately volcanic, has relatively good aquifers of considerable value for domestic, community and commercial water use.
  • Middle zone of 1,500-500M (amsl) which covers the Taita Hills, Shimba Hills and Arabuko Sokoke forest reserves, which are important water towers for the catchment. Chyulu Hills hosts the source aquifer that supplies the Mzima Springs, as well as the spring units that flow from the eastern side of the range (from Kiboko in the north to Umani further south). Volcanic aquifers on the northern flank of Kilimanjaro host a number of springs that are of good water supply, irrigation and tourism value (Nolturesh, Kimana and Entonet respectively).
  • Coastal zone of 500-0M (amsl) is threatened by seawater intrusion; worsening with proximity to the oceanic front. Where abstraction is limited, the coastal coral limestone and sand aquifer is of considerable commercial importance, and the Tiwi and Baricho Aquifers are key water supply. The coastal hinterland aquifers are typically poor and often brackish.


Water Resources in the Catchment

The Athi River flows from the southeast of Nairobi; north-eastward in the upstream reaches, and then turns its flow direction to the southeast in the north of Ol donyo Sabuk National Park, and flows along the catchment area boundary with the Tana Catchment Area, and pours into the Indian Ocean in the northern of Malindi.  The drainage area of the Athi River is 37,750 km2, or 64.4% of ACA.  The Lumi River, Lake Jipe, and Lake Chala flow into the territory of Tanzania and the Umba River reversely flows from Tanzania to Kenya.  Other rivers such as the Rare, Kombeni, Mwache, Pemba, and Ramisi rivers flow into the Indian Ocean and the total drainage area totals 19,493 km2. Territorial waters within Athi Catchment Area include Indian Ocean, covering a shoreline of approximately 210 km and extending into the sea 22Km or 12 nautical miles. There are several major springs in ACA such as Mzima, Kikuyu, NjoroKubwa, Nolturesh and Marere.


ACA is classified as a semi-arid land except in the upstream area of the Athi River which is classified as a humid land non-Arid and Semi-arid Land (non-ASAL).  The catchment has an annual average rainfall of 810mm. The annual rainfall differs spatially within the catchment area, ranging from around 500mm in the southern part near the border with Tanzania to 1,200mm in the western mountainous area. Evapotranspiration ranges from 1200-2500 mm per year in the catchment. Daily temperatures in the catchment area range between 10°C in the Upper Zone to 30°C in the Coastal Zone.

 ABA Water Demand by Subsector



Water Demands (MCM/year)
























Source: National Water Master Plan 2030

Land Use

  • Water towers, forest reserves and groundwater recharge areas: The Aberdare Range, Kikuyu Escarpment, Muguga, Dagoretti, Ngong Hills, Ololua, Karura, Kinare, Kiambu, Nyamweru, Kieni, Kitondo, Kilungu, Namanga Hill,Chyulu Hills, Taita Hills, Shimba Hills and Arabuko Sokoke
  • Wetlands and riparian zones: In Nairobi, Upper Athi, and Coastal Athi sub-regions, most wetlands and riparian land zones are rapidly being reclaimed and converted into residential estates or commercial/industrial centres. These wetlands and riparian zones need protection from degradation and restoration of their functional capacities.
  • Croplands and rangelands: Croplands and rangelands constitute the largest land use areas in the catchment. Rangelands in the Middle Athi and Nolturesh Lumi area support pastoralism. Unfortunately these rangelands are increasingly being converted to agro-pastoralism and urbanisation with consequent loss of the protective vegetation cover. Their fragile soil once disturbed by cultivation becomes susceptible to soil erosion during rainy seasons.
  • Urban areas: The major towns in this catchment are; Nairobi, Thika, Kiambu, Machakos, Kajiado, and Mombasa. They are associated with built-up infrastructure such as roads, air-ports and rail transport. With the availability of such infrastructure the area is characterized by significant economic activities such as manufacturing and processing industries, marine industry and Tourism. These areas generate high runoff and consequent erosion of riverbanks, sewage spills during flash-floods, solid waste, encroachment into wetlands and riparian.
  • Agriculture and pastoralism: The Upper Athi consists of the areas of Kiambu and some parts of Nyandarua, where farming of coffee and tea is predominant. Horticultural farming is also practised in these areas. Food crops like legumes, maize and fruits are grown in upper as well as middle Athi (Nairobi, Kajiado, Kiambu, Makueni, Machakos Counties and some parts of Kitui County).Other crops are cotton in Machakos County in the east of Nairobi.Cashewnuts, coconut and mangoes are grown in the coastal region of Kwale, Kilifi and Mombasa. Bananas are grown in Kiambu, Kwale and Taita Taveta Counties. Sisal is also grown in Makueni, Taita Taveta and Kilifi Counties.

Major Water Resources Management Challenges

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  •  Inadequate resources Inadequate water resources monitoring infrastructure
  • Vandalism of water resources monitoring infrastructure
  • Over abstraction of ground water in Nairobi and Mombasa
  • Flooding
  • Catchment degradation
  • Encroachment and cultivation of wetlands
  • Pollution mainly from major cities of Nairobi, Mombasa, Thika, Kiambu, Machakos and Athi River constituting major sources of pollution due to raw/partially treated domestic waste, industrial discharges and poor solid waste disposal.
  • Overgrazing causing accelerated erosion and subsequently siltation of water bodies.
  • Acute shortage of water resources at different times of the year resulting in conflicts of water sharing.
Figure 3:  Pollution sources along Athi River
Figure 3: Pollution sources along Athi River

WRA’s efforts in Curbing Water Conservation Concerns in Athi Catchment Area

  •  WRA has developed an electronic permit database (PDB) for Management of water permits, thus making access and payments more efficient.
  • ABA has carried out about 4 water abstraction surveys and developed 2 Water Allocation plans by June 2016 to determine the amount of water that has been committed through abstraction. This assists to determing the volume of water available versus the demand and hence plan to allocate the resources for various uses.
  • WRA Is in the process of Gazetting the Kikuyu springs as a Groundwater Conservation Area(GCA) to ensure the ground water recharge for the Nairobi aquifer suite is not polluted and depleted (currently drilling is being done at a depth of over 200m)
  • WRA launched a special program dubbed “Athi River Restoration Program” The Goal   of the   Program is to   protect water resources from  adverse impacts in collaboration with stakeholders for sustainable development. The Program will be implemented over a period of 5 years.
  • ABA has also carried out several activities aimed towards increasing the water storage through the installation of roof water harvesting tanks, spring protection, water pans and sand dams as a result of working with the WRUAs to address issues of water scarcity.
  • Empowered the WRUAs at the local level to carry out water conservation and protection activities through implementation of SCMP activities such as protection 129 springs, conservation of riparian areas, construction of terraces and gabions to control soil erosion, planting indigenious trees for conservation purposes.