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How does a waterfall form?

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Quick answer

A waterfall forms where a river flows from hard rock onto softer rock. The softer rock erodes faster, creating a step. Falling water and its spray then dig a plunge pool that undercuts the hard rock, causing it to collapse. This makes the waterfall edge retreat upstream, often leaving a gorge behind.

A waterfall forms where a river flows over a band of hard rock lying above softer rock, and the softer rock erodes faster, creating a steep drop. The falling water and its spray then undercut the hard cap, so the waterfall slowly retreats upstream.

Step by step

  1. A river flows over a layer of hard rock that sits above a layer of softer rock.
  2. The softer rock is eroded faster by the river's water and sediment, creating a step.
  3. The step steepens into a vertical drop as erosion continues to wear back the soft rock.
  4. Falling water and spray hollow out a plunge pool at the base of the drop.
  5. The plunge pool undercuts the hard rock cap until blocks collapse.
  6. The waterfall edge moves upstream, leaving a steep-sided gorge behind.

Key things to understand

  1. A river crossing from hard rock onto softer rock erodes the soft layer faster, carving a step in the channel.
  2. The step grows into a vertical drop as the soft rock is worn back by abrasion and hydraulic action.
  3. Falling water and spray hollow out a plunge pool at the base, undercutting the hard rock above.
  4. Blocks of unsupported hard rock collapse, and the waterfall edge moves upstream over time.
  5. Some waterfalls form instead along faults, where a sudden drop in the land creates the step.

Labelled diagram

  1. Hard rock

    A resistant layer that the river cannot easily erode, forming the cap.

  2. Soft rock

    A weaker layer below that is worn away faster by the river.

  3. Step forms

    Differential erosion creates a step in the river's long profile.

  4. Plunge pool

    Falling water and sediment carve a deep basin at the base.

  5. Undercutting

    The plunge pool erodes the soft rock behind the hard cap.

  6. Retreat

    Collapse of the cap moves the waterfall upstream, leaving a gorge.

How a waterfall forms and retreats

Real-world examples

  • Niagara Falls on the Canada–United States border has retreated several kilometres upstream since the last ice age.
  • Angel Falls in Venezuela drops from a flat-topped mountain called a tepui, where a resistant caprock forms the lip.
  • The Iguazu Falls on the Argentina–Brazil border formed where a river crosses a hard basalt layer over softer rock.
  • A small waterfall often forms where a stream crosses a band of resistant rock on a hillside.

Common mistake

✗ Myth: Waterfalls are permanent features that have always been in the same place.

✓ Fact: Waterfalls are temporary and slowly move upstream as erosion wears away the rock. Over thousands of years, a waterfall can retreat far from its original position, leaving a gorge behind, and may eventually disappear.

People also ask

What causes a waterfall to form?
A waterfall forms when a river flows over a band of hard rock lying above softer rock. The softer rock erodes faster, creating a step. The river then plunges over the step, and the falling water digs a plunge pool that undercuts the hard rock, causing collapse and upstream retreat.
How are waterfalls formed step by step?
First, a river crosses from hard rock onto softer rock. The soft rock erodes faster, making a step. The step steepens into a drop. Falling water and spray carve a plunge pool at the base. The pool undercuts the hard cap, which collapses, and the waterfall edge moves upstream.
Why do waterfalls retreat upstream?
The plunge pool at the base of a waterfall erodes the soft rock behind the hard cap. As the cap loses support, it collapses in blocks. Each collapse shifts the lip of the waterfall further upstream. This process continues for thousands of years, leaving a gorge below the fall.
Why do waterfalls move upstream over time?
The plunge pool at the base of the fall erodes the soft rock behind the hard cap. As the cap loses support, it collapses, and the lip of the waterfall shifts further upstream. This slow retreat can continue for thousands of years, leaving a gorge below the present fall.
What is a plunge pool?
A plunge pool is the deep basin carved out at the foot of a waterfall. The force of falling water and the grinding of rock fragments swirl it into a hollow. It is the main tool of erosion that undercuts the waterfall and drives its upstream retreat.
Can waterfalls form without two different rock types?
Yes. A waterfall can also form where a fault line drops the land suddenly, where a glacier has left a hanging valley, or where a river flows over a resistant band of rock. The common factor is a step in the river's long profile.
What happens to a waterfall as it retreats?
As the fall moves upstream, it leaves behind a steep-sided gorge. The river above the fall gradually cuts down to the level of the plunge pool. Eventually the step may be worn away, and the waterfall disappears, leaving a smooth river profile.

Part of the guide: Water & Weather Phenomena →