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How can industrial cooling water cause thermal pollution in lakes?

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lake Lake Ann in North Cascades National Park, Washington, U.S., viewed from the park's Maple Loop Trail. The North Cascades National Park is a large wilderness area that preserves majestic mountain scenery, snowfields, glaciers, and other unique natural features.

Industrial cooling water from power-generating plants, steel mills, and chemical facilities can cause thermal pollution in lakes by significantly altering the water's temperature, which negatively impacts the environment.12

When heat is discharged at the surface, it can raise local temperatures by several degrees, potentially exceeding the tolerance levels of fish and stimulating the growth of undesirable plankton and algae. If the waste heat is injected at depth or diffused over a large range, it reduces surface-level temperature spikes but decreases heat loss to the atmosphere, leading to a higher net heat input that can be more detrimental to the lake's ecology over time.1

Thermal pollution is considered a pollutant because it decreases the water's capacity to hold dissolved oxygen and increases the metabolic rates of fish. This combination can be fatal to sensitive game fish species, while indirect effects—such as excessive plant production—can lead to biological decay and further deoxygenation of the water.132