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.
Virginia watershed websites Chesapeake Bay watershed profiles - About 60 percent of the state drains to the Chesapeake Bay, the world's most productive estuary. Those watersheds include the Potomac (including Shenandoah), Rappahannock, York and James rivers as well as coastal lands on the bay's shoreline.