Sewage routinely contaminates the Hudson River, according to a report released on Tuesday after four years of water testing in which one-fifth of the water samples indicated that the river was unsuitable for swimming and other recreation.

The study, issued by the environmental group Riverkeeper, underscores how a big sewage discharge in July, caused by a fire at a treatment plant in Manhattan, was part of a persistent and far more widespread sewage problem along the 155-mile river.