What a watershed is

A watershed, or drainage basin, is all the land that sends its rain to the same place. A drop that lands anywhere in it runs downhill, joins a stream and leaves through one outlet: a river mouth, a lake or the edge of the map. The high ground between two basins is called the divide. Rain that falls a step to one side of it ends up in a different river, sometimes in a different ocean.

Why rivers join

Water always takes the steepest way down, and valleys funnel it together, so small streams meet and make bigger ones. Geographers count this with stream order: a stream with no tributaries is order 1, two order-1 streams that meet make an order-2 stream, two order-2 streams make order 3, and so on. On the model, rivers get wider as their order goes up.

How the model is made

1. Elevation is read for every cell of a grid over the area.
2. Hollows in the data are filled until they spill over, so water can always find a way out.
3. Each cell sends its water to its steepest downhill neighbour, and the cells are added up from the top down: that total is how much land drains through each point.
4. Where enough land drains through, a river is drawn. Following every path to where it leaves the area gives the basins.

In the classroom

Print your own valley and ask where a drop of rain on the school roof would end up. Pour a little water on the model and watch it follow the carved rivers. Compare the computed rivers with the real ones on the map, look for the divides, or switch on the rain-drop arrows to show which way the water runs on every slope. The table gives each basin's area, its longest flow path and the height of its outlet to compare.

What to keep in mind

The rivers are worked out from the elevation. With Follow the mapped rivers and streams on, the mapped channels from OpenStreetMap are cut a little into the elevation used for the flow (not into the model), so the computed rivers follow the real ones, which matters most on flat land. Turn it off to see where the terrain alone sends the water, and compare with the mapped streams drawn as thin dark lines on the basin map (canals and ditches are dashed). Without mapped streams, very flat land, such as wide floodplains and deltas, can give rivers that run straight or in parallel. In dry places where water collects in a basin with no outlet, the model treats the low spot as if it filled up and spilled over. Basins that cross the edge of the area are cut off where the map ends.

Printing it

With a multi-colour printer, load the 3MF and give each basin its filament and the rivers, lakes and sea blue. On a single-colour printer, print it in one colour: the carved rivers and the steps between basins still show, and you can paint the basins afterwards. The label strip prints flat; set it in front of the model.