The water has been tainted with human-made chemicals, sewage, pesticides and an abundance of bacteria throughout the years. Before the Clean Water Act was passed in 1972, factories based near the river used to dump their garbage directly into the water instead of a landfill.

The East River’s "gross" reputation stems from its history as an industrial dumping ground and its status as a tidal strait, not a true river. For over a century, factories and gas plants lined its banks, leaching toxic chemicals and heavy metals into the sediment. Even in 2026, the primary issue is New York City’s Combined Sewer Overflow (CSO) system; during heavy rains, the city's sewers exceed capacity, and a mix of rainwater and untreated "blackwater" (sewage) is discharged directly into the strait. Because it is a tidal strait connecting the Long Island Sound to the Atlantic, the water churns with powerful, turbulent currents that trap debris and pollutants rather than washing them away cleanly. While the water quality has improved significantly since the 1970s, the high levels of bacteria after rainfall and the "legacy" pollution in the riverbed mud keep it from being a safe place for swimming or direct contact.