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.