Surface runoff refers to any water that flows over the surface of the Earth and into lakes, rivers and other bodies of water. This is in contrast to the water that is absorbed by soil or plants.
Surface runoff is a crucial part of the water system. Water evaporates from lakes and rivers, but it must be replenished. While some rain or snow will fall directly into bodies of water, they also need some of the water that falls on land in order to maintain their size or flow. Surface runoff does carry the potential of sweeping up pesticides and other waste products and then polluting a small body of water.