The geological setting of the Thiririka River catchment is strongly influenced by the development of the East African Rift. The Thirika River has its source in the eastern parts of the Aberdare Range. The Range was developed by tertiary and quaternary volcanism on the eastern edge of the Great Rift Valley, the Kenyan part of the Gregory Rift.
On the contrary to the western flank of the Aberdare Range, the eastern flanks are gently sloping. The narrow valley of the Thiririka River is running in East-south-eastern direction. It is deeply increased and the slopes are steep. In the West, the altitude reaches around 2700m, the outlet of the catchment is located at an altitude of around 1400m.