Aim of the study:
This study presents an opportunity to determine a value of longitudinal dispersion and is focused on the comparison of seasonal variation of the longitudinal dispersion predetermined by natural conditions of the river aquatic zone.

Material and methods:
The dispersion of a pollutant tracer was simulated by HEC-RAS model and by one dimensional advection-diffusion equation. Determination of longitudinal dispersion coefficient was done on the base of tracer experiments (2 kg NaCl in 10 l H2O) carried out in March, August and October 2019. Both methods were applied to the Okna River localized in the Eastern Slovak Lowland

Results and conclusions:
Result showed, that model HEC-RAS very sensitive reacted to the changes of dispersion and vegetation in the river. Estimated longitudinal dispersion coefficient for Okna river by equation during the year, was in a range from 0.093 to 1.08 m2s-1 and by the HEC-RAS it was from 0.220 to 1.850 m2s-1. Results could represent different values of longitudinal dispersion simulated by the model or equation. Obtained coefficients have a wide application. A main use of these numbers is with simulation of the spread of accidental pollution in rivers. Also, it can be applied to streams with similar characteristics. Results showed important role of seasonal variation of longitudinal dispersion coefficient that must be assumed in simulation of pollution spreading in rivers.