The Athi Basin has an area of 66 559 km2 and borders Tanzania to the south, the Indian Ocean coastline to the east, the Tana Basin to the north and the Rift Valley Basin to the west. The topography of the Athi Basin varies from sea level at the Indian Ocean coastline to approximately 2 600 masl in the highlands of the Aberdare Range.
The Athi River is the main river in the Athi Basin, draining about 57% (38 170 km2 ) of the basin. It is the second longest river in Kenya (after the Tana River) and has a total length of 390 km. The Lumi River originates in the Athi Basin in Kenya along the eastern slopes of Kilimanjaro, and flows across the border into Tanzania, while the Umba River flows from Tanzania into the Athi Basin south of Mombasa. Various smaller rivers drain the eastern coastal parts of the basin and discharge into the Indian Ocean. The Athi Basin includes major wetland systems including Olngarua swamp, Ol Keju Ado, Ndumato, Mamanga Esokota, Jipe, Mangeri Swamp and the mangrove swamps along the coast. The Athi basin also includes inland freshwater lakes such as Jipe, Chala and Amboseli.
From the upper reaches of the Athi River to the Tsavo confluence, the vegetation types in the upper and middle Athi Basin transition from forest to woodlands and savannah. The lower part of the Athi Basin is dominated by savannah type vegetation. The Athi Basin is endowed with a wide range of forest ecosystems ranging from the montane forests in the Aberdares, savannah woodlands in the Taita Taveta, Machakos, Makueni areas and the dryland coastal forests.
The total population of the Athi Basin is 13.43 million, which is equivalent to a population density of 202 persons/km2 . The Athi Basin is the basin with the highest ratio of urban to rural population.
Land use in the Athi Basin includes urban, industrial and agricultural use. However, many areas are characterised by unsustainable land use.