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.