Empirical data on the behavior of radionuclides in aquatic ecosystems are needed for radioecological modeling, which is commonly used for predicting transfer of radionuclides, estimating doses, and assessing possible adverse effects on species and communities. Preliminary studies of radioecological parameters including distribution coefficients and concentration ratios, for ⁹⁰Sr and ¹³⁷Cs were not in full agreement with the default values used in the ERICA Tool and the RESRAD BIOTA codes. The unique radiation situation in the Techa River, which was contaminated by long-lived radionuclides (⁹⁰Sr and ¹³⁷Cs) in the middle of the last century allows improved knowledge about these parameters for river systems. Therefore, the study was focused on the evaluation of radioecological parameters (distribution coefficients and concentration ratios for ⁹⁰Sr and ¹³⁷Cs) for the Techa River and the Miass River, which is assumed as a comparison waterbody. To achieve the aim the current contamination of biotic and abiotic components of the river ecosystems was studied; distribution coefficients for ⁹⁰Sr and ¹³⁷Cs were calculated; concentration ratios of ⁹⁰Sr and ¹³⁷Cs for three fish species (roach, perch and pike), gastropods and filamentous algae were evaluated. Study results were then compared with default values available for use in the well-known computer codes ERICA Tool and RESRAD BIOTA (when site-specific data are not available). We show that the concentration ratios of ¹³⁷Cs in whole fish bodies depend on the predominant type of nutrition (carnivores and phytophagous). The results presented here are useful in the context of improving of tools for assessing concentrations of radionuclides in biota, which could rely on a wider range of ecosystem information compared with the process limited the current versions of ERICA and RESRAD codes. Further, the concentration ratios of ⁹⁰Sr are species-specific and strongly dependent on Ca²⁺ concentration in water. The universal characteristic allows us to combine the data of fish caught in the water with different mineralization by multiplying the concentration of Ca²⁺. The concentration ratios for fishes were well-fitted by Generalized Logistic Distribution function (GLD). In conclusion, the GLD can be used for probabilistic modeling of the concentration ratios in freshwater fishes to improve the confidence in the modeling results. This is important in the context of risk assessment and regulatory.