DRAGON SCIENCE: Dams and Dolphins
amble on the future for the Chinese. It is hoped that what will be the world's largest dam will control the flooding that occurs frequently on the river. But will the dam have more adverse consequences? No one can say for several decades. Meanwhile, the freshwater river dolphin (the baiji) that inhabits the Yangtze is seriously endangered. Construction of the dam raises critical issues about development, the environment and ecology.
Why build a dam? Most large dams are built to generate electricity. By interfering with the natural flow of water, dams produce an artificial lake or reservoir. Water that amasses within the reservoir rises and gains potential energy. When allowed to pass through the dam, the water falls to a lower level. As it drops, the water's potential energy is changed into kinetic energy.
A pinwheel-like device called a turbine harnesses this energy. Water striking the turbine's blades causes the turbine's shaft to rotate. This rotational movement is transferred to a nearby generator. Within the generator, the spinning motion is changed into electrical energy. This energy is then sent out to an electrical power grid along high voltage transmission lines.
Activity 1: Freshwater Woes
Activity 2: Measuring Sedimentation Potential
Activity 3: Build a Turbine
For Further Thought
ACTIVITY 1: FRESHWATER WOES
As in most organisms, the concentration of salt in the cells of dolphins is similar to that of ocean water. In ocean water, cells of an organism are in balance inside and out. But when the organism is placed in fresh water, an imbalance is created, resulting in osmotic pressure. Fresh water pushes through the semipermeable cell membranes in an attempt to dilute the salt concentration in the cells. If the freshwater dolphin does not get rid of the excess water through excretion, cells and tissues will swell. Like many species, the baiji has adapted to the stresses of freshwater life.
- 2 dialysis shells
- 4 paper clips
- 1 molar sucrose solution
- 2 large beakers
- plastic wrap
In the following activity, you'll observe the presence and effects of osmotic pressure.
- Fill two dialysis shells with 1M sucrose solution.
- Twist and secure the ends of the filled shells with paper clips.
- Place one shell in a beaker of fresh water. Place the other shell in a beaker filled with 1M sucrose.
- Cover both beakers with plastic wrap. Set aside out of direct sunlight.
- On the next day, record your observations of the dialysis shells.
- What did the 1M sucrose solution represent?
- Which beaker represented the Yangtze water?
- What happened to the dialysis bag in each beaker?
- What caused any observable difference in the shell's appearance?
- Why didn't the sugar molecules flow out of the shell?
- Cell contents.
- The beaker filled with fresh water.
- In the fresh water, it swelled. In the sucrose solution, it stayed the same.
- Osmotic pressure produced by the fresh water forced water into the shell, causing it to swell.
- The shell is made of a semipermeable material that restricts the movement of large molecules.
ACTIVITY 2: MEASURING SEDIMENTATION POTENTIAL
A dam interferes with the normal flow of sediment in a river. When blocked by a dam, water slows. Suspended particles no longer obtain the energy needed to remain afloat. Instead, they sink and accumulate as sediment that may fill in reservoirs or clog drainage and turbine pipes. One anticipated impact of the Yangtze River dam is excess buildup of sedimentation.
You can measure the relative amount of sediment within a water column by using a device known as a Secchi (rhymes with techie) dish. Although simple in design, the Secchi dish (along with its hi-tech equivalents) is used by many marine and freshwater scientists. As the dish sinks, its image becomes obscured. When the dish can no longer be seen, a measurement of the depth to which light penetrates is taken and used as a relative measure of the water's particulate load.
- round metal disk, approximately 8 inches in diameter with small
- central hole (old LPs will work)
- black and white waterproof paint
- cord (6 yards or more)
- eye bolt with two nuts, one on top, one on bottom (or more, to give added weight as needed)
- waterproof markers
- meter stick
Measure and compare sedimentation in a nearby waterway.
- Drill a small hole through the center of the disk and smooth any rough edges.
- Paint the pattern as shown below on the top of the disk. Allow to dry.
- Pass the eye bolt through the upper portion of the disk and fasten with a nut. Tie one end of the cord to the eye bolt.
- Use a meter stick to mark off 10cm divisions on the cord.
- With instructor supervision, lower the disk into water. Stop when the disk reaches the disappearing point. Note the mark on the cord.
- Compare this with measurements taken on other days and/or measurements taken from other bodies of water.
ACTIVITY 3: BUILD A TURBINE
You can build your own turbine system using a small DC motor, a digital multimeter and a pinwheel constructed out of ordinary materials.
- small DC motor (from toy or hobby store)
- source of flowing water
- construction materials (aluminum foil, cardboard, paper clips, pinwheel)
- digital multimeter
This challenge will help you visualize how a turbine and generator work on a dam.
- Attach the leads of a multimeter to the terminals of a small motor.
- Spin the motor's shaft. Observe and record any changes detected by the multimeter.
- Spin the shaft in the opposite direction. Describe any observed differences.
Work in small groups on this activity. Your group's task is to design a turbine system that will transform the energy of falling water into electricity. You are limited to the construction materials supplied by your instructor.
FOR FURTHER THOUGHT
- A dam can have a major impact on the environment. Some consequences of dams include: altering the natural balance of the river, limiting nutrient input associated with flooding, interfering with migration of wildlife, creating habitats that breed pests and parasites, displacing large human populations, flooding critical habitats. Imagine you are the engineer in charge of a project to build a dam on a local river. How would you address each of the issues listed above?
- Investigate the impact of other dams such as Egypt's Aswan Dam and dams on the Columbia and Snake Rivers in the American Northwest.
- How does a society balance the needs of its people with those of other species?
- At what point do the efforts to save a species become counterproductive?
- What do you predict will be the ultimate impact of the Three Gorges Dam?
Scientific American Frontiers
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