The Potomac River is cleaner than ever before due to efforts to reduce pollution. In the 1960s, 50% of fish had liver tumors and most oysters were contaminated by fertilizers draining into the river. Now, the bottom of the river has more grass and debris from pollutants is reduced at the surface. Scientists take water samples, and people volunteer to help clean the river, all helping to make the waterway less polluted for wildlife.