Why do fast-moving rivers cut deep valleys?

Rivers begin high up in the mountains so they flow quickly downhill eroding the landscape vertically. Hole this helps!!

"Rivers begin high up in the mountains so they flow quickly downhill eroding the landscape vertically. The river cuts a deep notch down into the landscape using hydraulic action, corrasion and corrosion".

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a substance in a physical state in which it does not resist change of shape and will expand indefinitely to fill any container.

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