Comment by burnished

Quick chemistry lesson; when you dissolve a solid into liquid it becomes more soluble as the temperature increases. Temperature equates to the energy with which atoms move around, so the solid can sort of shake free of its pattern and fall into solution. Liquids and gases have the opposite relationship where higher temperatures means lower gas solubility. Gases are already free, so when the temperature is lower its more likely that the bonds of the liquid are stronger than the gases propensity to bounce around.

Which brings us to heat pollution; heating a river will cause the water to lose oxygen (which it already does not carry much of nor very well). Anything that depends on that oxygen will suffer as a consequence.