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Geography

Why Playfair's Law Relates Stream Gradients to Valley Slopes

Quick fact

Playfair's Law states that tributary streams join main rivers at concordant junctions, where the tributary's gradient is adjusted so that its mouth meets the main river at the same elevation—so smoothly that they appear to have been fitted together, which is only true in steadily eroding landscapes.

Why this is interesting

Imagine hiking in a mountain valley: every side stream seems to join the main river at just the right level, without a waterfall or drop. Why do these streams always meet so smoothly?