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.
A watershed is a basin-like landform defined by highpoints and ridgelines that descend into lower elevations and stream valleys. A watershed carries water shed from the land after rain falls and snow melts. Drop by drop, water is channeled into soils, groundwaters, creeks, and streams, making its way to larger rivers and eventually the sea.