Water Erosion

In this lesson, students focus on how erosion shapes Earth’s surface. They use that knowledge to analyze how the matter that makes up rocks is shaped and reshaped by the rock cycle. This in turn transforms Earth’s surface over time. 

• Earth’s surface can only change with enough energy and that moving water has energy that shapes the land as it moves over it.  
• All water on Earth’s surface is pulled on by Earth’s gravity. This attractive force pulls water downhill toward Earth’s center.
• Weathering and erosion are two processes that shape the land by breaking down Earth’s materials into smaller pieces of sediment and transporting that sediment to new locations.
  
  

• Why does water on Earth’s surface flow downhill?
• Why does all moving water have energy?
• Why is the Colorado River considered one of the most powerful rivers in the world?
• What is the relationship between the slope of a river and its energy?
• How do people use the energy of the Colorado River?
• How does the Colorado River carve out a canyon as deep and wide as the Grand Canyon?  
• How is weathering different from erosion? How do weathering and erosion work together?
• What is the relationship between the solid rock layer of Earth and sediment?  
• How can wind and water cause tall mountains to transform into a flat plain over time?  
  
  

Misconception: Earth’s surface is static and does not change.

Fact: Earth’s surface is constantly changing. Plate tectonics, weathering, and erosion are three processes that change Earth’s surface over time.

Erosion: the transport of sediment by wind, water, or gravity

Rock cycle: the processes that form, break down, and re-form rock from one category to another

Rocks: mixed mineral matter that makes up the surface of Earth and other terrestrial objects

Sediment: a collection of particles of rocks, minerals, and the remains of plants and animals

Weathering: the breakdown of rock into smaller pieces from exposure to wind, water, gravity, changes in temperature, and/or biological forces

The Colorado River

The Grand Canyon would not exist without the Colorado River. It is because of the river that the canyon is as wide and deep as it is. It is also because of the river that we can see all of the layers in the rocks of the canyon.

The Colorado River is 2,330 kilometers (1,450 miles) long and 91 meters (300 feet) wide. It begins in the Rocky Mountains before it reaches the Grand Canyon. It gets its name from the Spanish word for red color. The river used to look red because of the red sandstone sediment of the land. Sediment is a collection of particles of rocks, minerals, and the remains of plants and animals. It collects in layers over time.

As the Colorado River moves, it shapes the land underneath. It carries with it pieces of the red sandstone it flows over. It does this through weathering and erosion.

Weathering is the breakdown of rock into small pieces from exposure to wind, water, gravity, changes in temperature, and/or biological forces (such as plants or animals). Wind, water, or gravity then transports the sediment to new locations in a process called erosion.

Weathering and erosion often work together. For example, wind and water carry pieces of sediment that grind against rock and wear it away. Water can also seep into cracks between rocky particles and expand, making cracks larger. Over time, the rock wears down from the inside and crumbles into sediment, which then gets carried away.

How Does a Mountain Become Flat?

Weathering and erosion are so powerful that they can cause tall mountains to become flat over time. Weathering and erosion can also cut through the land, carving out a canyon so large it can be seen from space.

How can moving water do this? The Colorado River is one of the most powerful rivers in the world. It is so powerful that it can carry large pieces of sediment — including large boulders —in its flow.

The Grand Canyon is so deep because the rocks carried along in the river act like tools. They chip off pieces of the riverbed as they are carried along. This process causes large amounts of rock to be weathered and eroded. Scientists estimate that 1 billion tons of rock have been carved out of the land to form the Grand Canyon.

In this lesson, students carry out an experiment to observe how the steepness of a slope affects the amount of sand eroded by water. Students use the information they gather from the experiment to construct an explanation about how water erodes sand when it moves over a steep slope, a slight slope, or no slope.