Thermal pollution-thermal means heat. interestingly the wastewater generated by factories and cities is generally warmer than the usual temperature of the lakes and rivers. when these wastewaters mix with the lakes and rivers they raise their temperature. with the rise in the temperature of water, several algae and waterweeds start growing rapidly. these organisms consume a large amount of oxygen from the water. this increased oxygen demand (i.e. the amount of oxygen available vis-�-vis the number of organisms) on the water causes serious threats to marine life.