The dead zone on the river ran from Philadelphia to about 25 miles down river in Marcus Hook, Pennsylvania. That made it impossible for migratory fish like shad to breed. They would die on their journey upstream before they could lay their eggs in the upper Delaware. Once plentiful caviar and sturgeon also disappeared.

The "dead zone" on the Delaware River refers to a historically significant area of severe water pollution and hypoxia (a lack of dissolved oxygen) that primarily affected the tidal reaches of the river near Philadelphia, Camden, and Wilmington. During the mid-20th century, particularly around World War II, this section of the river was essentially treated as an open sewer for industrial and municipal waste. The pollution was so extreme that it created an "oxygen block" or "dead zone" where dissolved oxygen levels dropped to nearly zero, making it impossible for aquatic life to survive. This zone was particularly devastating for migratory fish like the American shad, as it prevented them from swimming upstream to spawn, nearly wiping out the population. Since the formation of the Delaware River Basin Commission (DRBC) in 1961 and the passage of the Clean Water Act in 1972, billions of dollars have been spent on wastewater treatment. While the "dead zone" is largely a thing of the past and oxygen levels have significantly improved, scientists in 2026 continue to monitor the area closely to ensure that rising temperatures and nutrient runoff do not lead to a recurrence of these lethal low-oxygen conditions.