The ENN Basin borders Ethiopia in the north, Somalia in the east, Rift Valley Basin in the west and Tana Basin in the south. The Basin covers an area of about 210 000 km2 , which is approximately 36% of the total area of Kenya. The population density is as low as 18 persons/km2 . Major urban areas within the basin include Nanyuki, Isiolo, Marsabit, Nyahururu, Maralal, Moyale, Mandera and Wajir.
The topography of the ENN Basin varies from 150m above sea level at the Lorian Swamp to Mount Kenya’s peak at 5 199 m above sea level. Most of the Basin lies below 1 000 m above sea level. The ENN Basin is divided into 27 sub-basins, 5AA to 5J. The Ewaso Ng’iro River is the largest river of the Basin, with a catchment of 81 750 km2 or about 39% of the basin. It originates from the high-lying areas around Mount Kenya in the south-western part of the basin. Its main upstream tributaries include Ewaso Narok, Nanyuki, Isiolo, Osinyai and Milgis rivers.. The lake is the source of the Ewaso Narok River which supplies water to Nyahururu and recharges the Ol-bolossat aquifer. Other wetlands in the catchment include Ewaso-Narok and Suguta Marmar swamps.
The Lorian Swamp is a large wetland that is home to many large mammals and other wildlife. Two periods of rainfall occur during the year, namely the long rains between March and May, and the short rains from October to November. The main area susceptible to flooding in the ENN Basin is Isiolo, Archer’s Post, Wajir, Mandera, Laikipia and parts of Garissa County. The Shaba, Buffalo Springs and Samburu National Reserves are situated along the Ewaso Ng’iro River while a large area of the slopes of Mount Kenya is designated as a National Park.
The dominant land uses in the Basin are rangelands although there is agriculture in the upper and lower basin. The total population of the ENN Basin is 4.10 million according to Census 2019, which is equivalent to a population density of 20 persons/km2 . Most of the population in the ENN Basin resides in rural settlements, with only 24% of the population being located in urban areas.