The changes in water runoff quantity are accompanied with changes in the quality of the water. In the forest, some leaf litter and soil will be picked up by runoff and washed into the stream. However, the quantity is small as there is less runoff to carry the material and it must flow across areas with many tree roots that can interrupt the flow and cause the materials to be deposited on the land. On agricultural fields, water running over the surface of unprotected soil can move or erode soil and carry it into the local stream. Herbicides and pesticides as well as the nutrients in fertilizers will also move with the soil and water. Under extreme conditions, a major gully can form on the land surface, making use of the land difficult.
 Rill Erosion creates small channels where soil is carried away |  Gully erosion produces a deep valley making the land unusable |
In urban areas, the runoff can contain soil from construction sites as well as oil, grease, dust, lawn fertilizers, and metals such as lead and mercury that have settled on the pavement from air pollutants associated with traffic, power plants, and the burning of fuels for other industrial uses. Definitions of some of these major pollutants can be found on this server.
