In this monograph authors clarified that the annual throughflow of the Latvian river Daugava has certain relationships, researched and found them also on other 50 most important rivers in the world, which opened a possibility to advance a well-grounded hypothesis of the causes of the phenomenon. The goal of the work is, based upon the experience accumulated in the management of energy systems and HPPs, to make clear the significance of the latter under the conditions of open market. Having ascertained that there was a need to improve the forecasting methods as far as the hydro energy production is concerned, it was necessary to verify the possibilities of such methods with the help of the energoinformatic discipline.