Introduction. This article is dedicated to isotope-hydrological studies of the moraine-dammed lake “Tamozhennoye”, located on the territory of the Central Altai (South-Chuya ridge) in July–August 2023. Purpose of the research. The purpose of the research was to determine the fraction of glacier meltwater, snow meltwater and precipitation to the feeding of a periglacial lake in the middle of the ablation season. Materials and methods. The main method of this research was the isotope analysis of natural water (determination of the relative concentrations of deuterium and oxygen 18).Water samples were taken from streams flowing in and out of the lake, as well as precipitation, glacier ice and snow. After that the isotopic separation of hydrographs was made. Results. According to the results of this study, the average isotopic characteristics of the components of the inflow and outflow of the studied lake were determined. In addition to isotope sampling, a complex of hydrological and meteorological observations was carried out. The changes of the water isotopic composition in streams flowing into and out of the lake was analyzed, which showed a significant effect on the runoff of snowmelt and precipitation. To determine the fraction of various components to the lake’s feeding, a three-component isotopic separation of the inflow and outflow streams hydrographs was performed. In addition to the relative oxygen 18 concentration (δ18O), deuterium excess was used as the second isotope tracer. Discussion. It was determined that in the summer of 2023, the average fraction of glacier meltwater to the feeding of Lake “Tamozhennoye” was 49%. Despite its proximity to the glacier, more than 51% of the lake’s feeding in 2023 was due to snowmelt (24%) and precipitation (27%). To assess the correctness of isotopic separation, the calculation of the depth of runoff of various components was performed based on the results of the separation of hydrographs, which were compared with the observed data on ablation, precipitation and remote data on the area of snowfields. The comparison showed that the isotope method is well applicable for the runoff separation in the conditions of the Central Altai. Conclusion and resume. It was concluded that the studied lake “Tamozhennoye” in conditions of abundant precipitation and a large area of snowfields in the catchment area is characterized by a significant decrease in glacial feeding, which is typical for lakes of a quasi-stable stage. Suggestions for practical applications and directions for future research. The results of the study can be used to understand the features of the hydrological regime of the Altai moraine lakes and to predict dangerous hydrological phenomena associated with outbursts.