Abstract <p>The effect of the Chulyshman River on the formation of the density stratification pattern of Lake Teletskoye in summer is considered. The relevance of this problem is determined by the importance of studying the specific features of the transport processes of solutes and pollutants under the conditions of thermocline and thermal bar formation and water renewal in the lake. The above-mentioned problems are solved using mathematical modeling methods and field observation data. To consider thermohydrodynamic processes in deep Lake Teletskoye and the Chulyshman River, computer models were built based on the open Delft3D package: an improved 3D model was developed for the lake, and the flow of the Chulyshman River in its mouth area was described based on a planar (2DH) model. The 3D and 2DH models were matched at the inlet section to Lake Teletskoye. Field observations of the spatial thermal structure of the lake were carried out in the summer–autumn months of 2010–2023. The main results of the work performed include the revealed complex picture of density stratification and density currents in Lake Teletskoye during the period of summer heating of the reservoir as well as the main factors of its formation; features of the distribution of waters of the Chulyshman River in the lake in the summer months.</p>