A hydrological and hydrochemical database (produced by the M-TROPICS critical zone observatory) in the upper Nyong Basin from 1998 to 2017 was used to evaluate the river's response to climatic and anthropogenic forcing and examine chemical weathering processes. SiO and HCO constitute about 85 % of the Total dissolved solids (TDS) load, equivalent to 0.12 × 10 kg. y. Electrical conductivity (EC), Total dissolved solids (TDS), major cations, major anions (except F and NO) and alkalinity (Alk) vary seasonally and follow a predictable model with discharge. Atlantic Meridional Mode oscillation controls the long-term water chemistry. Atmospheric input and silicate weathering are the main factors influencing the Nyong rivers chemistry. However, several indices supported the progressive water quality deterioration by human activities, namely: the excess of Cl and SO after the substraction of atmospheric inputs, the basic pH observed for specific samples, long-term increase in the values of pH, EC, TDS, EC, Mg, Ca, F, NO, HCO, Alk, SiO and Dissolved Organic Carbon. Runoff and physical erosion have an important control on chemical erosion in the upper Nyong Basin rivers. The chemical erosion rate (3.3 t.km.y) equals the silicate weathering rate. The CO consumption rate, in the Nyong rivers, is lower than the global average (98× 10 for silicate weathering and 246 × 10 mol.km.y for chemical erosion) and estimated at 52.3 × 10 for silicate weathering and 54.1 × 10 mol.km.y for chemical erosion. At Olama, the most downstream location of the monitoring setup, the Nyong River Basin consumed 1 × 10 mol.y of CO by chemical erosion.