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Activity: Silt: A Four Letter Word

This lesson plan can be adapted for students in grades K-6. Students work in groups of four to create a model that simulates what happens to a stream and its oxygen supply when silt is added to the water.

Background

Two salmon

In this activity, students will experience what happens to oxygen when silt and sand are added to water. Salmon lay their eggs in gravel that receives a moving supply of clean water, either from a flowing stream or river, or from spring water percolating up from the lake bottom. As the water flows over the salmon eggs, it delivers dissolved oxygen to the eggs. If eggs do not receive enough oxygen, they die.

Silt and sand enter streams through erosion, which is most often caused by human activities. They act like concrete to block water movement, and thus diminish the amount of oxygen. Once the erosion-causing activity is stopped, streams may cleanse themselves. (Depending on the extent of the problem, self-cleansing can take from one to fifty years.)

Materials

For each group of students:

  • clear plastic or glass container such as a storage box or casserole dish
  • pea-sized gravel, four cups
  • water
  • coarse sand, one cup
  • silt (silica powder from the edge of a stream), one cup
  • straws, three per person

Objectives

The student will be able to:

  1. Describe how sand and/or silt effects oxygen in water.
  2. Identify human activities that add sand and/or silt to surface
  3. Describe their reactions to the experiment in a journal or essay.

Procedure

  1. Before class, set up one demonstration — a container with gravel covered by water — so that students can see how to proceed. Post a large sheet of paper on the wall for groups to record their results. (See illustration below)
  2. Illustration of results
  3. Place students into groups of three or four. Ask each group to gather their supplies and set up their demonstration.
  4. Each person in the group should blow bubbles into the water with a straw. The group then discusses its results and records its observations on the master sheet on the wall.
  5. Instruct the groups to add sand to the water, and then blow bubbles again. The group then discusses its results and records its observations on the master sheet on the wall.
  6. Now add silt to the water, and blow bubbles again. The group then discusses its results and records its observations on the master sheet on the wall.
  7. Conduct a whole-class discussion about the demonstration and results; and what these results might mean to salmon nests and their need for clean water.

Extensions

  • To increase the challenge and learning opportunity of this activity, you may wish to have the students research the conditions that salmon eggs require for incubation, and how siltation might affect those conditions. If necessary, specify a length of time for the research and set a time for reporting to the entire class.
  • Visit a nearby stream or lake and challenge the students to find examples of erosion or restoration.

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