Runoff (surface runoff)
Runoff is an important part of the water cycle. When rain falls from the sky, some of the water is absorbed into the ground and some of the water flows with gravity, usually into a ditch, sewer, lake, stream, river or another body of water. The amount of water that is absorbed into the ground is dependent on the type of surface covering the ground. In a forest, for example, the ground is mostly soft soil and leaf material, which can absorb more water. On paved streets and sidewalks, however, the rain cannot be absorbed, so much of the rain that falls in a city or residential area will become runoff.
Runoff can easily carry pollutants into major bodies of water. City streets often have a huge amount of automobile pollution. Farms may use pesticides and fertilizers, and if they have livestock then there is also a lot of animal waste on farmland. When the rain falls, it washes away a lot of these things, but they don’t just disappear. The runoff might end up in a nearby river, and with it will be the pollution that comes from the farms and the city. Not only can some of these chemicals be harmful to living creatures (including humans!), but it can also be harmful to ecosystems by introducing too much of a certain type of nutrient (like phosphorus or nitrogen in fertilizers). This can severely disrupt the balance of an ecosystem by allowing some species to thrive at the expense of others.