Industrial and mining activities are important sources of water-quality degradation and heavy metal contamination in river systems. This study aimed to assess the spatial and temporal variations in water quality and mercury (Hg) and cadmium (Cd) concentrations and their association with industrial and mining activities in the Talar River, northern Iran. Seven sampling stations were selected around the Shormast Industrial Park, Central Alborz Coal Mine, and downstream sections of the river. Sampling was conducted in September 2025, March 2026, and April 2026. Physicochemical parameters, including pH, electrical conductivity (EC), total dissolved solids (TDS), dissolved oxygen (DO), and oxidation–reduction potential (ORP), were measured in situ, while Hg and Cd concentrations were determined in the laboratory. Data were analyzed using analysis of variance, Spearman correlation, principal component analysis (PCA), and hierarchical cluster analysis (HCA). The results showed that pH, EC, and TDS were within acceptable ranges for irrigation water, whereas Hg and Cd exceeded the permissible limits of FAO, EPA, and ISIRI standards in some samples. EC and TDS showed significant spatial variations, while Hg and Cd exhibited significant temporal variations. The most pronounced changes in general water-quality indicators occurred downstream of the Shormast Industrial Park, where increased Hg concentrations, particularly in winter, suggested a potential influence of industrial activities. Increased metal concentrations were also observed downstream of the coal mine during some periods. However, the non-uniform Cd pattern and the presence of metals at upstream stations indicated contributions from natural and hydrochemical processes. Multivariate analyses identified the Shormast area as the most prominent zone of water-quality alteration. Overall, the findings highlight the need for continuous heavy-metal monitoring, industrial wastewater control, and mine-drainage management.