Optimum water flow is inevitable for river health protection. Urban rivers around Dhaka city, Bangladesh, is heavily polluted by numerous chemical pollutants including heavy metals. This study investigated heavy metals concentration and river-flow volume to elucidate the relationship between them. River-flow and metals analysis (n = 672 water samples) was performed for two consecutive years (November, 2017- October, 2019) in both the dry- and wet-seasons. The metal was analyzed by AAS, and Water-flow was measured by the formula of ‘velocity dimensional’. It was observed that, in the dry-period (November to May), the water-flow of the river ranged from 12.76 m³/sec to 49.87 m³/sec, whereas, in the wet-period (June to October), it was 35.39 m³/sec to 183.48 m³/sec. In the dry-period, the concentration of Cr, Cd, Pb, and Ni was found 3,860%, 12,900%, 8,200%, and 3,300%, respectively higher than the permissible limits. Optimum water-flow, ≥49.87 m³/sec, can run-down these metals from the river both in the dry- and wet-periods to retain environmental balance throughout the year. The proposed modeling of the optimum water-flow to minimize heavy metals concentration would be a promising measure to tackle the risk of the world’s urban rivers.