What is runoff?

Runoff refers to the process where water flows over the surface of the land, typically after rainfall or snowmelt, and is primarily driven by the saturation of soil. When the soil becomes saturated, it reaches a point where it can no longer absorb additional water. As a result, any excess water will move across the ground, which is precisely what runoff entails. This movement can lead to various environmental impacts, such as the transport of pollutants into aquatic ecosystems.

Other options describe different water movement processes but do not accurately define runoff. For example, while water flow from snowmelt can contribute to runoff, it is not synonymous with the concept itself. Similarly, water evaporating from surfaces refers to the phase change from liquid to gas, which does not involve the surface flow characteristic of runoff. Finally, water accumulating in rivers is more about collection and storage rather than the process of water flowing from saturated soils over land. Understanding runoff is crucial in the context of aquatic pollution, as it can carry contaminants from urban or agricultural areas into natural waterways.