Water quality criteria for temperature are based on the time of the year. In addition to having its own toxic effect, temperature affects the solubility and, in turn, the toxicity of many other parameters. Generally the solubility of solids increases with increasing temperature, while gases tend to be more soluble in cold water.
Temperature is a factor in determining allowable limits for other parameters such as ammonia.
An important physical relationship exists between the amount of dissolved oxygen in a body of water and its temperature. The figure below shows the relationship. Water temperatures, even miles from the release points, may rise dramatically. The result may be dead fish, fish eggs that won't hatch or a total change in the fish population as warm water varieties replace the original trout or other cold water fish. Reproductive events are perhaps the most thermally-restricted of all life phases.