- 2-3 water samples collected from different rivers
- 2-3 5-gallon pails for holding water samples
- Secchi tubes – enough for each group to have one tube
- Paint or ice cream bucket with rope attached to handle
- Plastic cups or jars to transfer water from pails to tube (one liter works well)
- Datasheets for recording transparency, appearance rankings, and source of water sample
- 4 dowels or sticks (about 2.5 feet long) for stirring water samples
- Camera for taking pictures of collection sites
- Non-mercury thermometer for measuring stream temperature
How to choose sampling locations
Choose two to three separate stream locations to monitor. For a variety of transparency readings, choose locations that have different water clarity and watershed characteristics.
Locate one site on each stream, upstream of their confluence (where the streams join). Be sure each site is far enough upstream of where the two join, to avoid water backing up in the event of high water.
Locate a third site downstream of the confluence, ensuring that the river has had time to mix. The water should look consistent, not clear on one side of the stream and darker on the other.
In the Twin Cities, samples for a good teaching example can be collected from the following sites:
- Mississippi River upstream of confluence with Minnesota River (Hidden Falls Park)
- Minnesota River at Fort Snelling State Park boat launch
- Mississippi River downstream of confluence with Minnesota River (Crosby Farm Park)
Timing of sample collection
Samples should be collected on the same day at all sites and under similar conditions. For a short experiment, a one-time sampling may be sufficient. For a longer experiment, monitor weekly and then after rainfall events (typically greater than 0.5” inches of rain, but varies by stream) to determine the role runoff plays in stream transparency.
Activity (10 minutes)
Work in pairs (or groups of 4*). Have students pair up/separate into groups and line up behind the
sample buckets. Have the first pair/group in each line start the activity.
- Person 1 — holds the tube and removes the mini Secchi disk by pulling string out.
- Person 2 — stirs water (if applicable) and fills cup with water. Pour water into tube.
- Person 1 — looks down through the water. Their job is to lower the mini Secchi disk into the tube until it disappears, then raise it until it reappears. Repeat until they are comfortable with the midpoint or disappearance/reappearance. Then, pinch string against the top of the tube to hold the disk in place.
- Person 2 — looks where the disk sits on the tape measure on the side of the tube to determine the transparency to the nearest centimeter. Record this value on the datasheet.
* If working in groups of 4: person 1 pours the water — person 2 lowers the Secchi disk — person 3 reads the measure — person 4 records on datasheet.
Note: these discussion points are geared toward a 3rd–6th grade audience.
Before the activity:
- Talk about how measuring water transparency to check the health of rivers is like a doctor doing a blood test to measure the health of a person.
- Ask if anyone can define transparency – go over meaning.
- If conducting a long-term project - Ask the students to develop a hypothesis regarding what they expect transparency to be at the different sites.
Discuss general background on rivers
- What lives in a river? Hit big three – plants, “bugs” (macro invertebrates), fish.
- What do plants need to live? Sun, soil, water, food.
- How far can light travel in water? As far as you can see into it.
- What do bugs eat? Other bugs, plants.
- What do fish eat? Other fish, bugs, plants.
- How can light affect all of these? Will plants grow if they don’t get light? What will bugs eat if the plants can’t grow? What will the fish eat if there are fewer bugs and plants?
- Get general consensus that light is important.
Talk about the Secchi tube
- Measure the light that can travel in the water (transparency)
- Show the pattern on the mini Secchi disk and the tape measure (in centimeters) on the side.
- How to use the Secchi tube
After the activity:
- Ask for results from students –what transparencies did they get?
- Discuss amount of light that gets to bottom of river
- Would many rooted plants grow in rivers where samples were collected?
- Could there be other plants (i.e. algae and floating plants such as duckweed)?
- Will there be bugs?
- Will there be fish? What kind of fish would we find (walleye/northern vs. carp)?
- Walleye must catch their food, if they can only see 10 inches, will they catch their food? Not very well.
- Carp suck their food off the bottom like a vacuum cleaner (discuss specialize mouth placement on bottom of face and barbels – “whiskers” – used to sense food) – do they need to see their food to catch it? No.
- Which fish are more likely in water with low transparency? Carp, Bullhead, Catfish, etc.
- Which fish are more likely in high transparency water? Walleye, Northern, Trout, etc. (predators that need to be able to see distance in water to catch their prey)
- Ask students what kind of soil they have at home in their yards (fine silts and clays – very small or sandy soil – larger particles?)
- Ask who has made Kool-Aid — if you stop stirring, does it all stay mixed? No, the sugar settles out.
- Does the water still stay colored? Yes, colored small particles stay mixed. The river works the same way, the large particles like the sugar (sand, gravel) sink to the bottom, but the colored part (small clays) stays mixed.
- Ask if any kids have dirt driveways or sandboxes. When it rains, what happens to the water that runs off the driveway/sandbox? Is it clear or muddy? This helps to explain how runoff makes the river darker (more sediment filled) after a rain storm.
Discuss where students’ drinking water comes from (predominantly from Mississippi River in Twin Cities area). Remind them that people upstream used the water before they did, and that people downstream use it after them. Is it important to take care of the water? What can each of them do to help?
- Do not litter and pick up litter
- Plant grass/flowers/shrubs on bare soil
- Don’t waste water
- Fix leaks in cars (drains to river via storm drains)
Additional discussion for long-term projects:
- Compare transparency readings over time at the three sites – this could involve graphing the data using Microsoft Excel or some other software with graphing capability.
- How does transparency differ between the sites?
- What might cause the difference? (Consider size of stream, land uses in watershed, stability of stream bank, etc.)
- Does transparency change at a given site over time?
- What could cause an increase or decrease in transparency?
- Does transparency change after a large rainfall? If so, how long does it take for the stream to recover (transparency to rebound)?