🌊 Perfect Conditions
Waterfalls need specific conditions to form: a river flowing over rocks of different hardness, with hard rock overlying soft rock. The contrast in resistance creates the perfect recipe for waterfall development.
River Processes and Landforms » Waterfall Formation
What you'll learn this session
Study time: 30 minutes
- How waterfalls are formed through river erosion processes
- The role of hard and soft rock in waterfall development
- Key erosion processes that create waterfalls
- How waterfalls change over time through retreat
- Real-world examples of famous waterfalls and their formation
- The landforms created by waterfall processes
🔒 Unlock Full Course Content
Sign up to access the complete lesson and track your progress!
Unlock This CourseIntroduction to Waterfall Formation
Waterfalls are some of nature's most spectacular landforms, created where rivers flow over steep drops in the landscape. They form through a combination of erosion processes acting on different types of rock over thousands of years. Understanding how waterfalls develop helps us appreciate the power of moving water to shape our landscape.
Key Definitions:
- Waterfall: A steep drop in a river where water falls vertically over a rock face.
- Hard rock: Resistant rock that erodes slowly, such as granite or limestone.
- Soft rock: Less resistant rock that erodes quickly, such as clay or sandstone.
- Plunge pool: A deep pool at the base of a waterfall created by erosion.
- Retreat: The gradual movement of a waterfall upstream as erosion continues.
The Formation Process
Waterfall formation is a step-by-step process that takes place over thousands of years. It begins when a river flows over an area where hard rock sits on top of softer rock layers.
Stage 1: Initial Erosion
When a river flows over different rock types, it erodes the soft rock much faster than the hard rock. This creates a small step or ledge in the riverbed. The hard rock acts like a protective cap, whilst the soft rock underneath is gradually worn away by the flowing water.
💧 Hydraulic Action
Water crashes into cracks in the rock, compressing air inside. When the water retreats, the air expands explosively, widening the cracks.
🪨 Abrasion
Rocks and pebbles carried by the river scrape against the riverbed and banks, wearing them away like sandpaper.
💥 Attrition
Rocks and stones in the river crash into each other, breaking into smaller, rounder pieces that can cause more erosion.
Stage 2: Undercutting
As the soft rock continues to erode, it creates an overhang of hard rock. The river cuts backwards into the soft rock, creating a notch or cave-like feature underneath the hard rock layer. This process is called undercutting.
The flowing water becomes more turbulent as it hits the step, increasing its erosive power. The combination of hydraulic action and abrasion works together to deepen and widen the channel in the soft rock.
Stage 3: Collapse and Plunge Pool Formation
Eventually, the overhang of hard rock becomes too heavy and unsupported. Gravity causes it to collapse into the river below. The fallen rocks are swept away by the current and the process begins again.
At the base of the waterfall, the falling water creates a deep pool called a plunge pool. The force of the falling water, combined with the swirling rocks and debris, erodes the riverbed to create this distinctive feature.
Case Study Focus: Niagara Falls
Niagara Falls formed about 12,000 years ago when glacial meltwater began flowing over the Niagara Escarpment. The falls consist of hard limestone sitting on top of softer shale and sandstone. The waterfall has retreated about 11 kilometres upstream since its formation, creating the Niagara Gorge. Today, the falls retreat at a rate of about 30cm per year due to continued erosion.
Waterfall Retreat
Waterfalls don't stay in the same place forever. Through the continuous process of erosion, they gradually move upstream. This is called waterfall retreat.
How Retreat Happens
The process of undercutting and collapse repeats over and over again. Each time the hard rock collapses, the waterfall moves a little bit further upstream. Over thousands of years, this creates a steep-sided valley called a gorge downstream of the waterfall's current position.
⌛ Rate of Retreat
The speed of retreat depends on several factors: the hardness of the rock, the volume of water flowing over the falls and human interference. Some waterfalls retreat several metres per year, whilst others move only centimetres.
Factors Affecting Retreat
Several factors influence how quickly a waterfall retreats:
- Rock hardness: Softer rocks allow faster retreat
- Water volume: More water means more erosive power
- Climate: Freeze-thaw cycles can weaken rock
- Human activity: Dams can reduce water flow and slow retreat
Landforms Created by Waterfalls
Waterfalls create several distinctive landforms as they develop and retreat over time.
Gorges and Valleys
As waterfalls retreat upstream, they leave behind steep-sided valleys called gorges. These dramatic landforms show the path the waterfall has taken over thousands of years. The Niagara Gorge is a perfect example of this process.
🌋 Plunge Pools
Deep circular pools at the base of waterfalls, carved by the force of falling water and swirling debris.
🏔 Gorges
Steep-sided valleys created as waterfalls retreat upstream, leaving a trail of erosion behind them.
🌃 Rapids
Fast-flowing sections of river created by fallen rocks and irregular riverbed surfaces downstream of waterfalls.
Case Study Focus: Angel Falls, Venezuela
Angel Falls is the world's highest waterfall at 979 metres tall. It formed where the Churún River flows over the edge of a tepui (flat-topped mountain) made of ancient sandstone. The waterfall demonstrates how resistant rock can create dramatic drops when rivers flow over plateau edges. The falls were formed by the river cutting through softer rock layers beneath the hard sandstone cap.
UK Examples
The UK has many excellent examples of waterfalls that demonstrate these formation processes.
High Force, County Durham
High Force is one of England's most spectacular waterfalls, where the River Tees drops 21 metres over the Whin Sill. This hard dolerite rock sits on top of softer limestone, creating the perfect conditions for waterfall formation. The waterfall has retreated upstream over time, creating a dramatic gorge.
Hardraw Force, Yorkshire
This waterfall shows how water can exploit weaknesses in rock layers. The Hardraw Beck falls 30 metres over limestone cliffs, having eroded through softer rock layers to create an impressive single-drop waterfall behind which people can walk.
Human Impact on Waterfalls
Human activities can significantly affect waterfall formation and retreat processes.
🏗 Dams and Control
Building dams upstream reduces water flow over waterfalls, slowing erosion and retreat. This can preserve waterfalls but also reduces their natural dynamism.
Tourism and Conservation
Popular waterfalls face challenges from tourism, including path erosion and pollution. However, tourism also provides funding for conservation efforts and helps people appreciate these natural wonders.
Climate change may affect waterfall processes by altering rainfall patterns and river flows, potentially changing erosion rates and retreat speeds.