Examples of the possessive form of plural nouns: actors - the actors' dressing rooms. bottles - the bottles' labels. children - the children's playground. doctors - the doctors' offices. eggs - the eggs' carton. friends - my friends' names. geese - the geese's pen.
This covered pool is really a good example of a watershed. The blue cover represents the watershed, the area in which precipitation that falls flows down-gradient towards the lowest part of the basin, the center of the pool cover in this case. Likewise, in nature, water flows towards a valley, river, or lake.
What is a Watershed? A watershed is the area of land that drains to a particular lake, river or other waterbody. The graphic above, from Michigan Sea Grant, shows how a typical watershed works. Imagine a raindrop falling anywhere inside that watershed; no matter where it lands, it will eventually find its way to the main body of water at the bottom. If it lands lands in another watershed, it will drain to a different waterbody.