4.4 Polluting our rivers with cold water
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3, 14, 18, 24

Years K-6: Science and Technology, HSIE
Years 7-10: Agriculture, Science, Geography 
Years 11-12: Geography, Biology, Earth and Environmental Science, Society and Culture

SOSE, Science, Geography

Futures, Interdependence, Thinking

Science, Thinking Processes, Geography

>Students will gain an understanding of cold water pollution and how it affects native fish and other aquatic life

1 hour 

Materials required
>Two clear cups or glasses per group
>One coloured ice cube per group (add food colouring to water before freezing)
>Four litres of cold (the colder the better) coloured water
>Four litres of warm water

Cold water discharged from the base of large dams like this can impact on aquatic wildlife for up to 150 km downstream. Photograph: Bill Phillips

Cold water pollution is less of a problem with smaller and more low-level structures. Photograph: Bill Phillips.

Industry and Investment NSW
Cold water pollution is caused by water being released into rivers from the outlets at the base of large dams. Between spring and autumn, the water stored in large dams can form two layers: a warm surface layer overlying a cold bottom layer. This process is called thermal stratification. 

Since many older dams are only equipped to draw water from the bottom of the dam wall, water that is much colder than the natural river temperature is released downstream, causing cold-water pollution. 

Cold water pollution is one important factor behind the reduction in the range and abundance of native freshwater fish species in parts of the Murray-Darling Basin. It is estimated that up to 3,000 kilometres of mostly lowland (as opposed to high country) river habitat in NSW alone is affected by cold water pollution.

The cold water pollution phenomenon. Source: http://www.dpi.nsw.gov.au/fisheries/habitat/threats/cold-water-pollution
The cold water pollution phenomenon. Source: http://www.dpi.nsw.gov.au/fisheries/habitat/threats/cold-water-pollution

At least 140 dams in NSW have a water depth of 15 metres or more that could stratify seasonally, forming a cold bottom layer of water. However, not all of these cause cold water pollution, because their outlets are configured to allow water to be released at different levels, or the quantity of water released may be relatively small. Nine dams are thought to cause severe cold water impacts (see below). 

Cold water released from dams has several impacts on a river system:

>natural temperatures can be depressed by 8-12°C in spring and summer
>annual temperature ranges can be reduced
>summer peaks in temperatures can be delayed.

Cold water pollution can affect temperatures in river systems for many hundreds of kilometres downstream from the storage. Initial research has indicated that cold water pollution can result in the death of juvenile native fish and reductions in growth rates.

Native fish, along with other aquatic animals such as turtles and frogs, are cold blooded. Their survival, growth and reproduction depend on the temperature of their environment. Cold water pollution can reduce the growth of fish and even reduce their chance of survival.

Native fish, along with other aquatic animals such as turtles and frogs, are cold blooded. Their survival, growth and reproduction depend on the temperature of their environment. Cold water pollution can reduce the growth of fish and even reduce their chance of survival. 

Fish will not breed if conditions are unfavourable. Even in the right season for breeding, and with appropriate rises in the river, breeding may not occur if seasonal temperature changes and thresholds are not reached. As a result of cold water pollution, native warm-water fish may fail to breed, they may breed late in the season, fish eggs may fail to hatch or the young may die or develop more slowly.

Young fish depend on zooplankton blooms as a food source. Low water temperatures can delay or prevent the development of these blooms, eliminating an important food source.

The lack of seasonal temperature variation has been found to reduce the numbers of macroinvertebrate species that would normally be expected in the warm summer period. Studies show that the eggs of mayflies do not develop if temperatures are too low as well.

Cold water pollution
Thermal stratification
Temperature-dependent spawning cue
Thermal shock

Lesson plan
1. Have students visualise a time they were swimming in a lake, dam or river. Have them describe what they have felt as they moved into deeper water. It gets colder the deeper you go. The top layer of water is nice but its colder below.

2. Demonstrate thermal stratification using the two clear cups or glasses, coloured ice cubes (add food colouring to the water before freezing) and cold (the colder the better) coloured water (use food colouring in the water).

3. Ask students what they think will happen when warm water is poured gently into a glass with cold coloured water? 
Then carefully pour a 1/4 cup of warm, clear water into the glass, trying to create two distinctly separate layers.

It's best if the warm
water is poured slowly in
from the side of a tilted
glass to avoid mixing.

4. Ask the students to speculate about what will happen when a coloured ice cube is placed in a glass of warm water? 
Then fill a glass with warm water and place a coloured ice cube in. 

5. Have the students observe and record what happens for five minutes. 

6. Discuss with the students why the coloured water sank? 

7. Explain convection currents. Could the students see convection currents in the glass? Explain. Why do they think convection currents might or might not be found in lakes?

8. Challenge the students with an investigation of what similarities and differences exist between the layered water and what they might find in regional lakes during the summer?

9. Why might thermal layering occur in lakes?

10. Ask students to pretend their cups are large dams. What water would come out of a dam that released water in spring or summer from a pipe low on the dam wall?

11. Ask students to theorise, and share with them, what this could this mean to fish, frogs, platypus, turtles (and the foods they eat) living below the dam. Remind students that fish, frogs and turtles are cold-blooded (exothermic).

12. Get the students to design a dam which could protect fish and other aquatic wildlife downstream from cold water pollution. 

Remind them that dams
are rarely full in spring
and summer when irrigation
and environmental water releases
are often provided so it's not as 
simple as blocking off the outlets
from the base of the dam and
letting water run over the spillway.

Have students explore the temperature profile in a local lake, river or wetland by submerging a thermometer 10 cm into the water and then lowering it on a string to a depth of one meter. Is there any difference for a water body that is flowing as opposed to one that is still ? If so, why?

Secondary pathway
For the chosen waterbody collect temperature profile data at several dates across the seasons. 
Using the data, create temperature profile graphs. If stratification is evident, label each of the layers (epilimnion, thermocline, hypolimnion – see Glossary).
After data are collected and analysed, the class can examine the temperature profile graphs in chronological order.
Have students consider the following questions for their final report:

>Does the waterbody remain stratified throughout the summer?
>What variables might affect whether or not the waterbody remains stratified throughout the summer?
>Why is this type of information important to people who monitor and manage or use our lakes?