Which of the following can be considered as an external force impacting water bodies?

Pollution runoff is considered an external force impacting water bodies because it involves contaminants from the surrounding environment entering the water system. This runoff can originate from various sources, including agricultural fields, urban areas, and industrial sites, carrying fertilizers, pesticides, heavy metals, and other pollutants. When these harmful substances flow into rivers, lakes, or streams, they alter water quality and can have detrimental effects on aquatic ecosystems, fish populations, and overall water health.

In contrast, natural rainfall is a component of the water cycle that generally replenishes water bodies without introducing adverse effects, while aquatic plants play a crucial role in maintaining water quality but are considered part of the aquatic ecosystem rather than external forces. Riparian vegetation, although beneficial for erosion control and providing habitat, also functions within the ecosystem and does not accurately represent an external pressure impacting water bodies in the same way that pollution runoff does.