8.6 Thermal Pollution

What is thermal pollution?

Thermal pollution is a harmful increase in water temperature caused by human activity. A major source is industrial and power plants, which withdraw water for cooling and then discharge it back into rivers or lakes at a higher temperature, disrupting the aquatic ecosystem it enters.

Effects on aquatic life

Warmer water holds less dissolved oxygen, stressing fish and other aquatic organisms that need oxygen to survive. Rising temperatures also speed up metabolism, increasing organisms' oxygen demand right when supply drops. Sensitive species may die off or migrate, reducing biodiversity and shifting community composition toward heat-tolerant species.

Other causes and solutions

Besides power plants, removal of riparian vegetation (shade trees along banks) and urban runoff from hot pavement raise water temperatures. Solutions include cooling towers or cooling ponds to lower discharge temperature before release, and restoring streamside vegetation to maintain natural shading.

Key terms

Thermal pollution

A harmful rise in water temperature caused by human activity, often from industrial cooling water discharge.

Main effect of warm water on fish

Reduces dissolved oxygen while increasing metabolic demand, causing oxygen stress and possible die-offs.

Riparian vegetation

Plants growing along a river or stream bank; their shade helps keep water temperatures cool.

Cooling tower/pond

Structures used to lower the temperature of power plant discharge water before releasing it into the environment.

Consequence of thermal pollution on biodiversity

Sensitive species decline or migrate, shifting communities toward more heat-tolerant organisms and reducing diversity.

Try an AP-style question

What is thermal pollution?

1. Toxic chemicals released into rivers
2. Human-caused warming of water
3. Nutrient runoff causing blooms
4. Water cooled by melting glaciers