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 the area of land that drains into a body of water. The watershed also includes the creeks and other water bodies in the area. The quality of a water source depends greatly upon the watershed. The contaminants in a watershed will end up in the surface water body. This is the reason that watershed management is important.