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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i only have few minutes according to the map above, which shows religious cultural regions, what two religions are most commonly practiced in africa? a. christianity and buddhism b. buddhism and hinduism c. islam and christianity d. hinduism and islam