**1. Introduction**

The Athi River drainage basin is the second largest basin in Kenya after the Tana River drainage basin [1]. Furthermore, the river traverses agro-ecological zones of diverse climatic characteristics and land use activities. The river draws most of its

headwaters from the Kenya Highlands (Kikuyu and Ondiri springs) flowing through the dry and semi-arid lands of Kenya and discharging its waters into the Indian Ocean, north of Malindi town.

In addition, the Kenya highlands are endowed with vast water resources but land use activity changes in these areas have led to water quality degradation in addition to affecting the flow characteristics of the rivers in the basin. Within the basin, there are two major urban and industrial centres from Kenya. Nairobi city, which lies at the upper catchment areas, and Mombasa, which is located on the southern outlet of the basin. Most of the upper-Athi river tributaries and the three main streams investigated (Ngong, Nairobi and Mathare) drain the Kikuyu escarpment through the city (now Nairobi County) and joining before the Kilimambogo hill (Donyo Sabuk) to form the main Athi-Sabaki River as shown in **Figure 1**.

The Athi River Catchment area borders the Tana Catchment area in the north and is located in the southern part of the country as shown in **Figure 1** and borders the Indian Ocean in the east, Republic of Tanzania in the south, and the Rift Valley in the west. In addition, the Aberdare Range, one of the Five Water Towers, lies in the northern edge of the area. The basin occupies a total area of 58,639 km<sup>2</sup> , which corresponds to 10.2% of the country's total land area. Based on the Kenya National Bureau of Statistics report [2] the recent Census 2009, population of the basin in 2010 was estimated at 9.79 million people or 25.4% of the total population of Kenya with a population density of 167 persons/km<sup>2</sup> .

**Figure 1.** *Athi River catchment basin and main tributaries.*

*A Critical Analysis of the Water Quality Impacts on Water Resources in the Athi River… DOI: http://dx.doi.org/10.5772/intechopen.99667*

The catchment basin varies in topography ranging from 2,600 masl in the Aberdare Range to the sea level in coastal area. The basin covers three zones in altitude, with the upper zone at 2600–1500 masl, middle zone at 1,500–500 masl, and coastal zone at 500–0 masl.

The upper catchment area (headwaters area) is the centre of commercial production with many industrial and agricultural activities, which discharges their waste waters onto the river system. This pollutes the water that is used further downstream for irrigation purposes and rural domestic water supply. The continued discharges of effluents into the river system are a major concern for sustainable agricultural farming in the downstream areas and hence affecting the peoples' livelihoods and food security.
