When a River Passes Seven Stones…

  1. Explain how water cleans itself by filtering through a riverbed.
  2. Ask students what “Water passes seven stones and cleans itself” means.
    • Which processes are meant by the saying that “A river runs through seven stones”? (the closest processes may be filtering and sedimentation, although perfect cleaning means the interconnectedness of physical, biological, and chemical processes).
    • What is filtering?
    • What is sedimentation?
    • Ask pupils how they imagine filtering water in a river. (Through gravel and sand at the bottom of a riverbed).
  3. Explain to students that they will do a simulation of the natural cleaning effect of a river through the bottom of a riverbed.
  4. Preparing a filtering experiment:
    • Mark a bottle about three-quarters of the way down and cut it in two. Remove the cap and reverse the top, which should now fit neatly into the smaller bottom section.
    • Put the small stones into the bottom of the reversed top of the bottle so that the hole is covered, but not completely blocked. Add gravel on top of the stones and then a layer of sand. Put a small stone right at the top.



    • Pour murky water onto the stone so that it can trickle down through the bottle. The water will collect in the removable bottom section.
  1. Observe the effect of filtering:
    • What effect has the filter (composed of stones, gravel, and sand) had on the water?
    • Can stones or gravel do this on their own? To answer this question, perform the activity with each separate material and measure the turbidity of the sample water before and after filtering. Help yourself with the Activity Water Transparency (visibility).
  2. Discuss with students: Is physical filtration the most important self-purification activity of a riverbed? (No, there are biological processes that are very important. The most intensive biological processes are going on with microorganisms in the bottom of a riverbed).



Activity partially adapted from Environmental Education Activities for Primary Schools – Suggestions for Making and Using Low Cost Equipment – Environmental Education Series 21. UNESCO International Center for Conservation Education, Cheltenham, 1992, Chapter 4 Water, Activity 4.10 Water Filters, p.64.