Drainage basins — Geography, 14–17 years
How water travels through a river system, from higher ground to the sea. Geography, 14–17 years.
One river, one connected area
A drainage basin is all the land where rainwater eventually reaches the same river, lake or sea. Small streams join to form larger channels, like branches joining a tree trunk. The high land around the edge is the watershed.
Why use the basin as a unit?
Rain does not respect council or national borders, but it does follow slopes. A drainage basin gives geographers a sensible area for asking where water goes and how an action upstream affects people downstream. It helps with flood planning, pollution control and water supply.
Following rain on a map
Imagine rain falling on a hill between two valleys. Step 1: water flows downhill, not across the ridge. Step 2: it enters a stream, then joins a river. Step 3: trace that river to its outlet; every slope feeding it belongs to its basin. A road over the ridge may cross from one basin into another.
The basin is not just the river
A common mistake is to draw the basin as a narrow strip beside the main river. That seems reasonable because the river is the visible feature. However, the basin includes every slope, field and town whose water reaches that river, including land far from the channel. The boundary is found by following high ground.
Managing a shared water system
A factory upstream can affect drinking water downstream, and paving a hillside can make flooding worse lower in the basin. River authorities therefore measure flow, test water and coordinate decisions across the whole basin. The idea is useful because causes and effects may be many kilometres apart.
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