The main purpose of this study was to evaluate the effect of treated sewage flowing from sewage treatment plants located in the basin of the Supraśl river on the concentration and load of metals in river waters and its main tributaries. Three measuring- control points were chosen, on the river and its tributaries, located near Gródek, Sokółka and Dobrzyniewo. Selected points were located behind the discharge of treated wastewater from sewage treatment plants respectively – Gródek, Sokółka and Bialystok. The samples of treated sewage and water were collected in a period from May to November, once a month in 2014. Each individual sample was examined for the content of dissolved form of the following metals: Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, Zn²⁺, Fe^2+/3+. After taking into account water flow of the Biała, Sokołda and Supraśl in every month, metals loads expressed in mg·h^-1, transported by the Supraśl and its tributaries waters were calculated. In the study monthly metals loads discharged into the Biała, Sokołda and Supraśl by sewage treatment plants in Białystok, Sokółka and Gródek were also calculated. The studies have shown the impact of metals load in treated wastewater on metals loads in waters of studied rivers based on the obtained correlation. Most of the searched relations between loafs of Pb²⁺ – r = 0,88; Cd²⁺ – r = 0,98; Fe^2+/3+ – r = 0,45; Ni²⁺ – r = 0,55; Zn²⁺ – r = 0,86 were obtained in case of wastewater treatment plant in Gródek and Supraśl waters. In the study period we observed a diversity in concentration of Cd²⁺, Fe^2+/3+, Ni²⁺ and Zn²⁺ in treated sewage and in river waters, which affected loads of this metals.