Watershed Scientific Concepts Watershed Scientific Concepts Surface Runoff Much of the water that returns to Earth as precipitation runs off the surface of the land, flowing downhill into streams, rivers, ponds and lakes. This water eventually reaches the ocean or large lakes. Surface runoff is important to aquatic ecosystems, since the water running off the land eventually supplies these systems. Water, though, is not the only thing that runs off the land.
Water also carries water pollutants and soil, both are deposited into aquatic ecosystems.
Infiltration Not all water runs off the surface of the land. When infiltration occurs, some of the water ultimately returns to the surface, while the rest remains below as groundwater. The various layers of soil act like a filter to help clean water. When water is badly polluted, however, this filtering system is not nearly as effective. In some cases, water moving through the ground during infiltration can even pick up pollutants present in the soil.