Sediment: Definition & Deposits

Instructor: Maria Howard

Maria is a teacher and a learning specialist and has master's degrees in literature and education.

Learn about what materials make up sediment and how it travels. Explore sediment deposits from deltas to sand dunes, and learn how sedimentary rock is created.

The Elwha River Problem

As any beaver can tell you, dams are rather clever. We build dams across rivers to create reservoirs, which hold water for household and agricultural use. These give us access to water when we need it, and some bigger dams harness the power of water to create electricity.

But what happens when dams are no longer useful? What happens when we remove a large, man-made dam? We often think about the consequences of building dams, but not what happens when you take them away.

The Elwha River in Washington state was a valuable resource for the Native Americans who lived near it for generations, and 87% of it is now a protected part of the Olympic National Park. The Elwha Dam, built in 1913, and the Glines Canyon Dam, built in 1927, were constructed at the beginning of the 20th century to provide power for the growing region.

Over time, both power plants grew increasingly obsolete, and scientists learned more about how the dams negatively impacted the salmon population, who were kept from swimming upstream. In 2011, crews working on behalf of the National Park Service began to remove the Elwha Dam, and in summer 2013, removal of the two dams will be complete.

Part of the Elwha Dam was removed so water could flow freely again after 100 years. Eventually, the entire dam will be taken away and only the river will remain.
Photo of the partial deconstruction of the Elwha Dam

One thing that scientists have had to consider since the removal project started was what would happen to the rocks and small pieces of rock called silt that had built up within the two reservoirs for almost 100 years. Where would all this material end up, and how would it affect the Elwha River?

What Is Sediment?

All of the natural material collected in the Elwha dams--the rocks, clay, solid animal and plant matter--is called sediment. Sediment ranges in size from a grain of sand to a rock too heavy for a human to lift. All this debris was collected as the Elwha River flowed downhill and ended its journey when it reached the concrete walls of the two dams.

A core sample of sediment shows how the materials naturally form layers based on weight.
Photo of sediment

There is so much sediment stored in the Elwha dams that some geologists predict the geologic consequences will be the equivalent of a '100-year storm.' While individual storms move a great deal of dirt and debris, the release of the built-up Elwha River sediment will be the equivalent of 100 years' worth of storm debris.

Now that the dam removal is nearly complete, we now know that 34 million cubic yards of sediment has been collected and will enter into to the Elwha River. If you need a visual for how much physical material that is, think of 3 million dump trucks piled high with dirt and rocks.

Where Does It Come From? Where Does It Go? (A Sediment-al Journey)

In order to understand where all that sediment came from and where it might all go, we have to think about how sediment is created. All land is subject to weathering and erosion, which breaks rocks and minerals down through the power of water, wind, salt and even changes in temperature.

For example, glaciers pick up rocks and freeze them to their icy mass through a process called plucking. Rocks are ground down as they scrape the earth's surface, just as the earth's surface is changed by moving glaciers. Wind moves sand, dirt and soil, while rivers like the Elwha pick up rocks and silt in their current and move them downstream.

When sediment settles in a particular area, it is called deposition. Sediment from glacier deposits form landforms named moraines, while wind in sandy coastal and desert areas creates shifting mountains of sand called sand dunes. The normal course of action for undammed rivers is to carry their sediment to another body of water, such as a lake, ocean or another river. Depending on their flow, rivers create deltas, large deposits of rich soil that build up at the end of a river over time.

To unlock this lesson you must be a Member.
Create your account

Register to view this lesson

Are you a student or a teacher?

Unlock Your Education

See for yourself why 30 million people use

Become a member and start learning now.
Become a Member  Back
What teachers are saying about
Try it risk-free for 30 days

Earning College Credit

Did you know… We have over 200 college courses that prepare you to earn credit by exam that is accepted by over 1,500 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Create an account to start this course today
Try it risk-free for 30 days!
Create an account