Grades 3–5 · Earth & Space
How can raindrops on opposite sides of a hill reach different rivers?
Two raindrops can land close together yet begin very different journeys. If they fall on opposite sides of a ridge, the ground may slope toward different streams. Gravity draws each drop downhill along its own route.
In the diagramA ridge separates two surface drainage routessurface divideriver Ariver BInvented surface divide; groundwater can differ.
The high boundary between these drainage areas is called a divide. On one side, runoff may join a river flowing east. On the other, water may join a river flowing west. No fence is needed to separate the routes.
A watershed is the land area that drains toward a chosen outlet. Small watersheds can fit inside larger ones as tributaries join a main river. Changing the chosen outlet changes which whole area is being described.
Water can also soak into soil or return to the air, so not every raindrop reaches a river. But surface slopes explain a powerful organizing pattern: nearby places can belong to different drainage systems, even across one hillside.
About this explanation
Starting knowledge: Know that gravity pulls surface water downhill and that small streams can join larger ones.
Sources
- U.S. Geological Survey — Watersheds and Drainage Basins