Within the framework of correlation analysis, the existence of a relationship between random variables, its shape, strength and direction are established. The aim of the paper is to perform a correlation analysis of natural inflows grouped into smaller time periods based on relevant hydrological data measured over a number of years at a single measurement site. The paper calculates symmetric matrices of Pearson correlation coefficients for mean monthly inflows and mean weekly inflows. The average values of the correlation coefficients of mean monthly and mean weekly inflows for consecutive periods, as well as for larger time steps, are analyzed. Finally, correlations between the natural logarithms of mean monthly and mean weekly flows are calculated, which are necessary when modeling inflows that are subject to a log-normal distribution. Differences in the average strength of relationships are observed with respect to the type of correlation relationship and the observation period. The results obtained are significant for the development of natural inflow data models for the purposes of optimization calculations.