**3. Hydrology of the Athi River drainage basin**

The Athi River basin is drainage basin number three (3) of the Kenya's drainage basins and is about 540 km long. The river drains a catchment area of about 70,000 km<sup>2</sup> (66,837 km<sup>2</sup> ) representing 12% of Kenya's total land area. The mean annual run-off is about 1294 <sup>10</sup><sup>3</sup> with annual rainfall of 550 mm translating to 19 m<sup>3</sup> s <sup>1</sup> mean annual runoff for the whole basin [6, 7].

The Athi River originates and drains the southern slopes of Aberdare ranges and comprises the southern part of the country east of the Rift valley. It covers large parts of Kiambu, Nairobi, Machakos and Makueni Counties. The river flows through a country of basement complex rocks, the areas being mostly semi-arid and subject to long drought periods. Its tributaries from the highlands flow in deep valleys close together, almost forming a parallel drainage system.

The main tributaries are Ruiru and Ndarugu. Other minor tributaries but of great hydrological importance are Ngong, Nairobi, Mathare, Mbagathi, Riara and Gitathuru. Ngong, Nairobi and Mathare tributaries drain the upstream areas and traverse the Nairobi city and its environs. After being joined by its tributaries, the Athi River flows down steeply in a series of falls and rapids in a metamorphic formation. It goes down reducing in capacity due to underground seepage as a result of geological configuration; e.g. 95 m3 s <sup>1</sup> above Kwaa, 65 m3 s <sup>1</sup> at Kibwezi (161 km) downstream, 40 m3 s 1 70 miles (113 km) above the confluence of Tsavo (Republic of Kenya, [8]). Further down, it becomes reinforced by Tsavo River from Mt. Kilimanjaro and Mzima springs of the Chyulu hills which provide dry weather inflows after where it changes its name into Galana or Sabaki and flows in a series of meanders over sandy beds until it drains its waters to Indian Ocean north of Malindi as indicated in **Figure 1**.

The river carries with it an enormous volume of suspended sediments because of erosion and other human activities in its upstream reaches. The total amount of sediments discharged by the river into the Indian Ocean is estimated at 2,057,487 tonnes/year as indicated in **Table 1**. The heavy sediment loads discharged into the Ocean are responsible for the highly colored beaches of Malindi, which have changed totally to brownish therefore affecting the tourism industry by polluting

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


**Table 1.**

*Suspended load (t yr.<sup>1</sup> ) and its volume of some selected streams in the Athi river drainage basin.*

the sand beaches around Malindi town. The heavy sediment loads further adversely affect marine life by reducing light penetration and fishery. This in turn affects local fishing activities and livelihoods by reducing fish catches and sales, which the local communities relies on. Kithiia [7], Aketch and Olago [10] and Mavuti, [11], highlighted an increasing trend in water quality degradation within the basin. This complemented the findings of the present study.
