Watersheds and runoff
The idea
A watershed is the land area that drains toward a common outlet. Gravity and topography shape drainage. Hard surfaces often allow less infiltration than permeable soil, increasing runoff. Watershed boundaries follow higher ground, not necessarily political borders.
Worked example
Rain on a parking lot may enter a drain and reach a nearby stream. Rain in a planted area may infiltrate. Both areas can belong to the same watershed even if they are on different properties.
Another application
Two neighborhoods separated by a ridge may drain to different streams even within one town. On a topographic map, follow the downhill direction on each side of the ridge to identify likely outlets.
A common misunderstanding
Watersheds are land areas, not just the rivers inside them. Activities far from a stream can still affect water flowing toward it.
Try it yourself
Use a local topographic map to identify slopes and likely drainage directions. Do not enter drains, floodwater, or restricted waterways.
Check your understanding
Would a watershed necessarily stop at a town boundary?
Reveal the explained answer
No. Its boundary is determined by the shape of the land.