Drainage basin

ASTER satellite image of branching tributary valleys near the Colorado River in Canyonlands, Utah. — ASTER, 24 September 2005: tributaries dissect sandstone in Canyonlands, Utah. A drainage network collects flow towards a main river.

A suggested lesson sequence

Use these stages at your own pace. Pause for students to sketch, compare observations or lead part of the demonstration.

01
Observe
Show the drainage divide and trace the edge of the basin. Identify which slopes drain towards the same outlet.
Open Observe
02
Tinker
Follow a tributary to its confluence with the main river. Hide the labels and ask a partner to locate the source, confluence and mouth.
Open Tinker
03
Explain
How does the drainage divide determine which slopes supply water to the same river outlet?
Open Explain
04
Check
Choose an answer and explain your reasoning. Each question includes feedback.
Open Check

Rehearse with the model

The same scene and controls as the lab. Load it when you are ready; use Start projector for the full classroom view.

Before the lesson

Trace water through a drainage basin and explain how the watershed connects the tributaries to a shared outlet.

Open the model before the lesson and choose a view that makes the important feature easy to see. Drag to rotate, and use the on-screen controls to change one variable at a time. The Controls panel explains the available keyboard shortcuts.

Pause the animation at a useful moment. Ask students what should happen next, then play it to test their explanation.

Guide the demonstration

Show the drainage divide and trace the edge of the basin. Identify which slopes drain towards the same outlet.

Follow a tributary to its confluence with the main river. Hide the labels and ask a partner to locate the source, confluence and mouth.

Discuss a common misconception

“All rain falling within a drainage basin reaches its river immediately.” Rainfall can be intercepted, infiltrate soil, replenish groundwater, evaporate or enter surface runoff. These pathways have different travel times, and some water remains stored or leaves through evapotranspiration. The river’s response depends on both rainfall and the basin’s existing conditions.

Build the explanation

A drainage basin collects water that flows towards a common outlet. The watershed separates it from neighbouring basins; tributaries connect hillslope inputs to the main river.

Follow a tributary to its confluence with the main river. Hide the labels and ask a partner to locate the source, confluence and mouth.

The terrain is a teaching catchment, and the flow traces show routes rather than measured travel times.

Teaching notes

Trace the full land area draining towards the outlet, not only the river channels.

Clarify whether watershed means the drainage divide or the whole basin in the terminology being used.

Distinguish the river mouth from a delta, which forms only where sediment accumulates under suitable conditions.

Check understanding

Open Check for 4 questions. Ask students to explain their choice before revealing the answer.

Return to the opening photograph. Ask students to identify one visible feature and explain which part of the model helps account for it.

Look more closely

Use these observations to connect the vocabulary to features students can identify.

Source

Melting snow patches and a stream near Fourth of July Lake in Idaho's Sawtooth Mountains. — Snowmelt near Fourth of July Lake, Idaho. Seasonal melting transfers stored snow water into streams, soils and groundwater.

Read the definition

Confluence

The Moselle joins the wider Rhine beside the city of Koblenz, Germany. — The Rhine–Moselle confluence at Koblenz, photographed in 1988. A tributary joins the main channel here.