The Tana Basin has an area of 126 208 km2. The basin borders Somalia and the Indian Ocean coastline in the east, the Athi Basin to the south, the Ewaso Ng’iro North Basin to the north, and the Rift Valley Basin to the west. Major towns in the basin include Garissa, Thika, Lamu, Nyeri, Embu, Meru, Murang’a and Kitui.

The topography of the Tana Basin varies from the peak of Mount Kenya at 5 200 masl and the Aberdares in the west at 3 900 masl to the coastal area at sea level. Most of the middle and lower parts of the basin is flat, while the upper basin is mountainous with steep slopes along the southern slopes of Mount Kenya and the eastern slopes of the Aberdares Range and Nyambene Hills.

The Tana River is the main river in the basin and drains approximately 76% of the Tana Basin. It originates on the slopes of Mount Kenya and flows eastward until it reaches Garissa town, after which it flows southwards and pours into the Indian Ocean at the Tana Delta. Upstream tributaries include the Chania, Thiba, Maragua and Thika rivers.

Eastwards-draining, seasonal tributaries join the main stem of the Tana River downstream of Garissa and include Nihunguthu, Maua, Komoli, Galole and Kokani rivers. Smaller rivers in the north-eastern part of the basin drain to Somalia and the Indian Ocean. The Tana River discharges into the Indian Ocean at the Tana Delta.

The vegetation cover in the Tana Basin is mainly savanna vegetation. The coastal areas are covered with mosaics of forests as well as tropical rainforest, swamp forest and mangrove areas.

Land use in the Tana Basin includes forest, grassland/rangeland and agricultural use. The basin has a high population density and scattered urban and built-up areas in the upper sections of the basin with the dominant land use as rain-fed agriculture and rangeland.

The total population of the Tana Basin in 2019 was estimated as 6.96 million, which is equivalent to a population density of 55 persons/ km2. Most of the population in the Tana Basin resides in rural settlements, with only 22% of the population currently being located in urban areas.