Run-off is when precipitation (snow, rain sleet, or hail) hits the ground and drains into rivers, lakes, and oceans. This run-off is what causes accumulations or build-ups of water. Run-off is also a cause of erosion, which can break down and carry rocks to different places. Millions of years ago, planet Earth did not really have rivers or lakes! Run-off from rain actually carved out the river channels and made rivers and lakes possible! Run-off is the leading cause of erosion, which can sweep away rock. That can damage the foundations of buildings and led to a lot of damage. That is why it is important to control run-off.