Methodical prerequisites are discussed of forecasting of effect of river flow divertion on relief and geomorphological processes. Some examples are given illustrating changes of relief and processes presumably resulting from divertion of the Pechora River. Possible changes of the relief and geomorphological processes due to inter-zonal re-distribution of water resources are to be considered using criterion of essential impact, i. e. reversible or irreversible disturbances of the dynamic equilibrium.