The use of mathematical models to assess river pollution and self-purification status is an important tool for effective surface water quality management.  The five day BOD test is widely applied to assess organic water pollution, and in defining the BOD ultimate value is adopted.  This study is conducted to evaluate the kinetics of BOD assimilation and its assimilative capacity along the Karu River.  Thomas slope method is used to evaluate the kinetic rate constant (k1) and the field measurements of hydrogeometric properties for finding the reaeration rate constant (k2) required for assessing the self-purification status of the river.  The result established the rate constant, K₁ for Karu River as 0.174 per day; with the lowest (0.124 day^-1) in the dry season and highest (0.223 day^-1) in the wet season which is within the range of internationally accepted values in practice.  The study concludes that the Karu River has a high self-purification potential, and the river is classified as a polluted river.  The effluent discharge into the river is below the National Environmental Standards and Regulation Enforcement Agency (NESREA), 2011 recommended limits.

KEYWORDS:  Karu  River,  Biochemical  Oxygen  Demand,  Rate  Kinetics,  Self-Purification.