Many industries (thermal power plants, oil refineries, and even nuclear power plants) use water as a coolant.
This wastewater after that comes after a cycle has a higher temperature (8-10°C higher) when compared with the intake water.
This wastewater is directly released into the nearby streams, rivers, or seas. This causes a sudden rise in the water temperature of these water bodies.
The aquatic life cannot withstand this high temperature (including both plants and animals) and get killed.