Forests play a key role in cleaning water. Precipitation is “filtered” through the tree canopy and filtered again through the organic matter on the forest floor. The water then seeps into the subsurface to replenish the ground water. Approximately 80% of the freshwater in the United States originates in the 650 million acres (265 million hectares) of forest that cover approximately one-third of the nation. These forested watersheds are the primary source of drinking water for nearly two out of three people in the United States. The United States’ national forests and grasslands, which cover nearly 192 million acres (78 million hectares), are the largest single source of freshwater in the country, and the headwaters of many large river basins originate in the national forests. Forests are thus an important source of freshwater; however, mining, forestry practices, and altered fire regimes have increasingly caused the contamination of these valuable water resources.