What is a watershed?

A watershed is defined as the area of land that is drained by a river. This concept is fundamental in environmental science because it encompasses not only the land directly adjoining the river but also all the smaller streams, creeks, and tributaries that contribute water to the main river. The boundaries of a watershed are determined by the topography of the land, particularly the ridges or hills that divide different drainage areas. Understanding watersheds is critical for managing water resources, as activities such as agriculture, urban development, and forestry can significantly impact the water quality and quantity that flows through the river system.

In the context of the other options, the underground layer that holds groundwater refers to aquifers, which are separate geological structures storing water. A structure used to control river flow represents systems like dams or levees, which are man-made interventions rather than a description of a geographical area. The percentage of a rock's volume that has spaces defines porosity, a property primarily related to geology and soil science, not directly about hydrological drainage areas. Hence, the correct option provides a crucial understanding of hydrology and the interconnectedness of land and water systems.