A study of the effect of subaquatic discharge of karst groundwater sources on the composition of the Vishera River, the largest tributary of the Kama River, was carried out. The study was carried out on the territory of the State Nature Reserve "Vishersky" and included the determination of the flow rate, temperature as well as chemical composition of natural waters. Six zones of karst groundwater discharge and their influence on the water regime of the Vishera river were studied in detail. It was shown that subaquatic sources in the places of their discharge, forming up to 36% of the river flow, significantly affect the temperature regime. A significant part of dissolved substances, primarily calcium ions and hydrocarbonate ions, as well as some trace elements (Li, Sr, V, Cr) enter the Vishery river with karst waters. The results of the study show that monitoring the composition and properties of groundwater discharged covertly in the river channel is an important part of the monitoring of water bodies of both protected and developed areas.