India, preferably the state of West Bengal is known for a number of religious festivals of which Durga Puja is perhaps the biggest. The tenth day of the Durga Puja, also known as Dashami is the day when Visarjan (immersion in water) of the idols occur with grand celebrations and processions. The idols are immersed from different ghats of the river. Along with the idols, puja articles such as flowers, food offerings, metal polish, plastic sheets, cosmetic items, polythene bags are also thrown into the water. The River Ganga, a sacred river originating from Gangotri in the Himalayas is the life-line of the city of Kolkata and hence any change in the water quality of the river is an issue of concern. The present paper is an attempt to assess the quality of the river before and after immersion of the idols during the Durga Puja festival of 2018. The increase and significant variations of dissolved heavy metals between pre- and post- immersion periods has been confirmed through ANOVA (p < 0.01). The variations in water quality (in terms of selective heavy metals) may be attributed to the immersion of idols, which are usually made up of Plaster of Paris (gypsum, sulphur, phosphorous and magnesium) and mostly painted with harmful synthetic dyes (contains mercury, chromium, cadmium and lead), which add heavy metals in the water body. Use of natural dyes sourced from flowers, leaves, seeds, bark, wood and roots of plants may reduce this hazard to a great extent.