How Much Sediment Can a Stream Carry?

Overview

This lesson was created by Rebecca Harris after being a part of the Arctic Glacial Lakes PolarTREC Expedition. She was inspired by how important suspended sediment, something so often overlooked by non scientists, was for developing paleoclimate models as well as ecosystems. Students will observe a watershed or a model of a watershed to make predictions about what might influence the amount of sediment carried by a creek or river. Students will then use an authentic data set and spreadsheets to calculate the total discharge and sediment carried by Carnivore Creek, one of the streams studied by the Arctic Glacial Lakes Project. Finally, students may use coarse field methods to estimate sediment carried by a local stream.

Objectives

• Students use an authentic data set to calculate seasonal discharge and total sediment.
• Students understand that greater discharge of a stream results in greater energy of the water and thus more suspended sediments.
• Students understand the importance of sediments in fluvial systems.

Lesson Preparation

• Teacher should use the provided photos (Lesson Materials) to become familiar with the field site.
• Teacher should become familiar with the data sheet and prepare it for students. Teacher should prepare student data sets with the yellow columns removed so students can derive their own calculations. Depending on technology available, teacher may wish to give each group a subset of the data to reduce the size of the spreadsheet. For example, each group analyzes one month of data, then the class combines calculations. The entire data set may run slowly on older computers!
• If possible, class should visit a local stream or river to make observations and field measurements (not required). A smaller stream is preferable, as it will be safer for students to sample.
• Students should have access to at least one computer per group and spreadsheet/graphing software: provided data will work in Google Sheets or Excel.
• For the extension, conducted at the local stream, bring several meter sticks or transect tapes, rulers (if you body of water is shallow) and a few liter jars with lids, such as mason jars.

Background information

It will benefit students and teachers to read the journal posts from the Arctic Glacial Lakes expedition that detail the science of the expedition. These posts are labeled “The Science.” The following journals will be the most helpful for this lesson.

1. The Science: Arctic Glacial Lakes details the data collection, models, and goals of the Arctic Glacial Lakes project.

2. The Science: Why Study Mud? discusses the importance of studying sediments, which will help students understand the purpose of this activity.

3. The Science: Glaciers will help students understand the importance of glaciers on hydrology and sedimentology.

Students should also have a good understanding of discharge, suspended sediment, and sediment flux.

Discharge is the volume of water traveling in a waterway in a given amount of time.

Suspended sediment is the mass of sediment carried in the water.

Sediment flux is the mass of sediment transported in a given volume of water over a unit of time.

Generally, the higher the discharge, the greater the energy in the water, and thus more and larger sediments will be suspended, yielding a greater sediment flux. Students should also understand that Julian date is a count of days in the year, which allows us to easily graph a continuous passage of time. Decimals in the Julian data give the time of the day.

Procedure

1. If possible, visit a local creek or river. Have students observe sediments that are suspended in the stream. Taking water samples using clear containers may help with this process. Also have students observe the substrate (bottom materials) and how they change in different locations. Things to watch for: more sediment suspended in faster water, grain size of substrate is larger where water is faster.
2. If a body of water is not accessible: have each student group fill a jar about one third full of soil, then fill the rest of the way with water. Ask students what they will expect to see when the jar is shaken. Students shake jar then put it on table, observing what happens over time. Things to notice: larger sediments settle first, the smallest last, the larger the sediment, the more energy it requires to suspend.
3. Back in the classroom, give students an overview of the Arctic Glacial Lakes expedition and field site, using the journal articles and photos provided. Focus on orienting students to Carnivore Creek, where this lesson’s data comes from.
4. Ask students to predict how much sediment travels in Carnivore Creek each season (spring through fall). What might influence this amount? Will the amount of sediment carried be different in different seasons? Why might winter be excluded from our estimate?
5. Define the vocabulary for the lesson. Suspended sediment concentration (SSC) is the mass of sediment in a given volume of water, discharge (Q) is the flow of a stream measured by volume of water in a given amount of time, and sediment flux is the mass of sediment transported in a certain volume of water over a given amount of time. Make sure students are aware of units (provided on data sheet).
6. Divide students into small groups. Provide the groups with the data set and the goal (to calculate the amount of sediment that is transported by Carnivore Creek in the season). Students calculate the sediment flux over the season. Depending on the mathematical and spreadsheet capabilities of the class, teacher may need to scaffold the lesson by providing more information, such as the highlighted notes on the teacher spreadsheet.

Extension

1. Make predictions about how the sediment flux of your body of water will compare to that of Carnivore Creek. Consider what factors influence flux.
2. Revisit the river or creek as a class. Bring meter sticks, rulers, a transect tape, and some liter jars.
3. Measure the cross sectional area of the body of water by measuring the width and several depths.
4. Calculate a rough estimate of flow by floating a small object (such as a leaf or and orange) in the current. Time how long it takes the object to travel one meter. Use this estimate to calculate flow (m/s).
5. Collect a few liter samples of water to measure sediment. When collecting the sample, try to move the jar from the surface of the water to close to the bottom (don’t disturb sediments on the bottom!) to sample the entire water column. Collect samples from the thalweg, which is the deepest part of the channel where water moves the fastest.
6. Back in the classroom, multiply cross sectional area (in meters squared) and flow (m/s) to calculate discharge.
7. Use a filter and filter paper (the chemistry teacher at your school should have these supplies) to filter the sediment from the water samples. Mass the dry filter paper before and after filtering to determine the mass of suspended sediment in one liter water.
8. Calculate a rough estimate of sediment flux for the year for your body of water. What assumptions are you making? Is your estimate to high or too low? What methodology would make your estimate better? How does sediment flux for your location compare to that of Carnivore Creek? Is this expected or unexpected? Why?

Resources

Data Set: Carnivore Creek and NG1 Data 2017

Photos from Google Earth

Journal Entries from PolarTREC website

Assessment

Students may also be assessed by the extension. Student groups determine methods for calculating an estimate of flux and collect data to find estimate with scaffolding from teacher as needed. Assess on appropriate data collection an accuracy of calculate estimate.

Author/Credits

Rebecca Harris created this lesson. Please email her at the.rebecca.harris [at] gmail.com if you have any questions, comments, or feedback. I would love to hear from you! Data was organized by PolarTREC researcher Ellie Broadman, and provided by the Arctic Glacial Lakes Project. All data is preliminary, and some may have been adapted slightly to be more clear and concise for students, but to maintain the general trends. Special thanks Darrell Kaufman, Nicholas McKay, Lorna Thurston, David Fortin, Erik Schiefer, and the many other students and field assistants who generously provided the data.