Study region: The Nyando River basin in western Kenya. It is part of the Lake Victoria basin. Study focus: As a first step towards numerical modeling, complementary datasets from different periods have been used to develop a conceptual hydrogeological model for this geologically complex basin. The study focused on characterizing the physical and process structures. Data from geological reports and geo-electrical survey reports were used to establish aquifer extents. Timeline comparison and interpolation of groundwater levels and hydrochemical data analysis were used to define groundwater flow characteristics. New hydrological insights for the region: A highland and a Kano Plains groundwater system exist and are separated by the faults at the Nandi and Kendu escarpments. The groundwater divide between Nyando and Nyamasaria Rivers flowing through the Kano Plains is indistinct, suggesting that their lowland groundwater system is the same. Kano Plains has a shallow and deep groundwater system, both occurring in highly heterogeneous and aquifers; the bedrock here occurs at >250 m below ground level. Generally, the groundwater flow is topography driven towards Lake Victoria, but it is not clear whether it discharges here. Rainfall has minimal effect on long-term groundwater variability. Because groundwater levels do not change significantly over time, steady-state groundwater flow occurs in the plains.