Effects of Sewage discharge on a river : 
(i) The micro-organisms in the sewage consume alot of oxygen and this pose a problem for other organisms in the river.
(ii) The Biochemical oxygen demand (BOD) of the river increases.
(iii) Presence of large amount nutrients causes excessive growth of planktonic algae or Algal bloom which imparts distinct color and degrades the quality of water and fish mortality. Some bloom-forming algae are extremely toxic to human beings and animals.
(iii) Various disease causing microbes  may cause outbreak of serious diseases, such as, dysentery, typhoid, jaundice, cholera, etc