Heat has two main effects. As mentioned when discussing natural organic pollutants, a rise in temperature brings about a decrease in the amount of oxygen dissolved in the water. At the same time, a rise in temperature increases the metabolic rate of organisms and therefore their demand for oxygen.
Heat can also cause fish to spawn and hatch out of season and to alter their migration patterns. To avoid such problems, when heated water is returned to rivers after being used for cooling (in power stations, for example), there should always be a minimum flow in the river to dissipate the heat sufficiently and so keep the temperature rise to only a few degrees.