Trace Drainage Through a Watershed

Use slope, stream connections, and divides to reason about runoff.

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A watershed drains toward a common outlet. Ridges can divide neighboring drainage areas. Trace connected downhill routes rather than assuming water follows the shortest distance across a map.

Worked example

Rain on opposite sides of a ridge can enter different streams. Two nearby places can therefore drain to different outlets even when a map places them close together.

An Invented Drainage Network

A ridge separates West Basin from East Basin. Stream W drains West Basin toward Lake W. Stream E drains East Basin toward Lake E. Along one 3 km reach of Stream W, elevation falls from 600 m to 300 m. The streams do not connect within this model.

1. Where does runoff entering Stream W travel within this network? Toward Lake E across the ridge Toward Lake W Automatically to whichever lake is closer

2. What role does the ridge play? It connects both lakes with a pipe It removes all runoff It separates drainage areas

3. What is the average elevation drop per kilometer along the stated reach of Stream W, in meters per kilometer? Answer

4. Would material carried downstream in Stream W necessarily enter Stream E in this model? Yes, all nearby streams connect immediately No, the streams are not connected within the model Yes, water always crosses ridges

5. Which information matters most when predicting the surface runoff destination of a new point? Its position relative to slopes, divides, and connected streams Only the nearest place name Only the color used to label it

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