The scientists, at the Stroud Water Research Center in Avondale, Pa., said the findings about streamside, or riparian, forests have large implications for the world's growing freshwater crisis. Currently, 20 percent of the planet's population lacks clean drinking water and more than 2.2 million people die each year from diseases transmitted by contaminated water and poor sanitation.

For some time, scientists and environmental policymakers have recognized the role that riparian forests play in filtering pollutants before they enter the stream. The new research shows, however, that such forests also protect the health of the stream itself by enhancing the ecosystem's ability to process organic matter and pollutants such as nitrogen.

Therefore, the scientists said, deforestation of riparian lands compromises both the quantity and the quality of a stream's ecosystem, reducing its ability to deliver important services to humans.

The study was conducted on small streams, which comprise more than 90 percent of all streams in the United States, so there could be enormous implications for improving water quality by planting trees along stream banks, they said.