The dam is owned by Athi and operated by the Nairobi City Water & Sewerage Company. The dams construction started in the year 1988 when the Kenya Government compulsorily acquired approximately 1,200 acres of land to create space for the construction of the dam to supply portable water to the residents of Nairobi City and its environs.
Water from the dam is conveyed through a system of tunnels running from the dam, tapping Kiama River and Kimakia River and diverting the flows to Chania River at the Mwagu Outfall. Water is tapped from Chania River at Mwagu and conveyed via a tunnel to the Mataara chamber from where it’s conveyed by pipelines to Ngethu water production plant.
The Dam construction was undertaken under the Third Nairobi Water Supply Project between 1989 and 1994. The contractor for the above contracts was m/s Strabag International and the Consultant Engineers were m/s Howard Humpreys (K) Ltd.
The project was co-financed by the World Bank, ADB, European Investment Bank and the Kenya Government among others. The estimated total cost of the project stood at Kshs. 2 Billion at that time.
Key Functions Of Thika Dam
• To harness water from the Dam’s catchment to impound Thika reservoir.
• To supply continuous reliable water to the City of Nairobi and its environs, with minimum interruptions, at regulated quantities.
• To release regulated quantities of compensation water downstream to maintain the natural Thika river course.
• To encourage a forestation within the Dam’s catchment and around the Dam, to sustain the catchments and ensure the presence of an environment for continuous water flow.
• To collect and analyze embankment instrument Data, using computer programmes to ensure the safety of the earth fill embankment.
• To carry out physical analysis on stored water to establish its quality.
• To monitor, collect Data and keep records of the weather changes around the Dam.
The dam has a storage capacity of 70,000,000m3 at full storage level of 2,041 meters above sea level and a depth average of 65m. The dam Produces 430,000 m3/day of water that is about 84% of total supply of water to Nairobi residents.
The Dam’s catchment area measures 75 square Kilometer. It consists of Kimakia and Gatare Natural forest which form Aberdare Ranges. The main rivers that drain into the Dam from this catchment are Thika, Githika and Kayuyu. Thika drains 50% Githika 30% and Kayuyu 20% of the catchment into the Dam respectively. The area receives ample rainfall of between 2000 – 2500 mm annually.