The appropriate operation of a power plant requires the rapid cooling of water steam and consequently large amounts of water. It has led to the construction of power plants on large rivers or near lakes. Barrages and channels had to be built in some places to deliver water for cooling. The threat of warming water and its impacts rives and lakes was considered. Consequently, studies on the effects of warm water on aqueous systems were necessary. The author presents results of such studies using Rybnicki Reservoir in Ruda (Silesia) as an example.