Explain the effect on the characteristics of a river when urban sewage is discharged into it.

Sewage from urban areas includes sewage from houses, hospitals and industries. Sewage from houses and hospitals may contain pathogenic bacteria which could lead to outbreak of diseases. Sewage from industries could pollute water with toxic materials, heavy metals etc which could be highly poisonous to aquatic lives. Thus the water of the river may turn dark with all the waste without any aquatic life.