peer reviewed ; As part of a broader study on the riverine biogeochemistry in the Athi-Galana-Sabaki (A-G-S) River catchment (Kenya), we present data constraining the sources, transit and transformation of multiple nitrogen (N) species as they flow through the A-G-S catchment (~47 000 km2). The data set was obtained in August-September 2011, November 2011, and April-May 2012, covering the dry season, short rain season and long rain season respectively. Release of (largely untreated) wastewater from the city of Nairobi had a profound impact on the biogeochemistry of the upper Athi River, leading to low dissolved oxygen (DO) saturation levels (36-67%), high ammonium (NH4+) concentrations (123-1193 μmol L-1), and high dissolved methane (CH4) concentrations (3765-6729 nmol L-1). Riverine dissolved inorganic nitrogen (DIN; sum of NH4+ and nitrate (NO3-); nitrite was not measured) concentration at the most upstream site on the Athi River was highest during the dry season (1195 μmol L-1), while DIN concentration was an order of magnitude lower during the short and long rain seasons (212 and 193 μmol L-1, respectively). During the rain seasons, low water residence time led to relatively minimal in-stream N cycling prior to discharge to the ocean, whereas during the dry season we speculate that prolonged residence time creates two differences comparative to wet season, where (1) intense N cycling and removal of DIN is possible in the upper to mid-catchment and leads to significantly lower concentrations at the outlet during the dry season, and (2) as a result this leads to the progressive enrichment of 15N in the particulate N (PN) pool, highlighting the dominance of untreated wastewater as the prevailing source of riverine DIN. The rapid removal of NH4+ in the upper reaches during the dry season was accompanied by a quantitatively similar production of NO3- and nitrous oxide (N2O) downstream, pointing towards strong nitrification over this reach during the dry season. Nitrous oxide produced was rapidly degassed ...