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Reading passage Grade 7 Science

How Dams Store Water and Make Electricity: A Hoover Dam Reading

A grade 7 informational passage on how dams store water for supply and flood control and generate hydropower, using Hoover Dam's real power capacity.

Objectives

  1. Students will identify the multiple purposes a dam and its reservoir can serve.
  2. Students will explain how a dam converts moving water into electricity.
  3. Students will infer why elevation drop and flow both matter for a dam's power output.

Vocabulary

Reservoir
A human-made lake, usually created by a dam, that stores water for later. Reservoirs supply cities and farms and help control floods.
Turbine
A machine with blades that spin when water rushes past, like a high-tech waterwheel. Spinning turbines turn generators that make electricity.
Hydropower
Electricity made from moving water, usually water falling through a dam. It is a renewable power source that can start up quickly when the electric grid needs it.

Also: generator, head

Teacher overview

Rivers have been dammed for centuries to store water, but in the last hundred years, many dams have taken on a second job: generating electricity. This passage explains how a single structure can hold back a river to supply drinking water, control floods, and spin turbines for power, using Hoover Dam on the Colorado River as a real, working example. It gives students a concrete case for reasoning about why a dam's location, not just its size, determines how much electricity it can produce.

How Dams Store Water and Make Electricity: A Hoover Dam Reading — Grade 7 Science
Name: ____________________________ Date: ______________

Passage

A dam is a wall built across a river. The lake that forms behind it is called a reservoir. The most common reason to build a dam is water storage. A reservoir holds extra water from wet seasons. That stored water gives a community a reliable supply during dry seasons. Dams also help control floods. Operators can hold back a surge of storm water and release it slowly afterward. This stops the water from rushing downstream all at once. Farms downstream often depend on this released water too. They use it to water crops during the growing season.

Many large dams do a third job: they produce electricity, called hydropower. Water in the reservoir flows out through openings near the dam's base. It rushes through machines called turbines. A turbine has blades that spin when water pushes past them. Each turbine connects to a generator, a machine that turns that spinning motion into electricity. Two things control how much electricity a dam makes. The first is how much water flows through the turbines. The second is how far that water drops before it reaches them, a distance engineers call head. A dam with a tall drop can make a lot of power, even without a huge flow of water, because each gallon falls farther and hits the turbine with more force.

Hoover Dam sits on the Colorado River, between Nevada and Arizona. It is one of the best-known dams built for both water storage and hydropower. Its reservoir, Lake Mead, is one of the largest reservoirs in the United States, and it supplies water to cities and farms in several states. Hoover Dam's power plant was built to generate up to about 2,080 megawatts of electricity, which makes it one of the larger hydropower plants in the country. Years of drought have lowered Lake Mead, though, and the Bureau of Reclamation reports that the dam now makes about 30 percent less power than it used to. Builders finished the dam in the 1930s, during the Great Depression. The project gave thousands of people construction jobs at a time when jobs were hard to find.

Large dams also come with real trade-offs. A dam blocks a river's natural path, which can stop fish such as salmon from reaching the upstream waters where they spawn. A reservoir's still water is very different from a flowing river, and that change affects the plants and animals that need moving water. Over time, a river's sediment settles at the bottom of the reservoir instead of washing downstream, which slowly shrinks how much water the reservoir can hold. Communities and engineers weigh these trade-offs against a dam's benefits: water storage, flood control, irrigation, and electricity. They weigh these factors whenever they plan to build, run, or remove a dam.

Questions

  1. According to the passage, name two things a dam and its reservoir can do besides producing electricity.
  2. Explain, in your own words, how a dam turns moving water into electricity.
  3. What is Hoover Dam's power plant capacity, according to the passage?
  4. Based on the passage, why might a dam on a steep river make more electricity than a dam on a flat river? Assume both dams have a similar amount of water flowing through them.
  5. Name one trade-off or downside of building a large dam, according to the passage.
  6. Engineers look at both a dam's water drop and its water flow to estimate electricity output. Why do both factors matter? Use evidence from the passage.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: Water storage and flood control (or irrigation).
    The passage explains that a reservoir stores extra water from wet seasons for use in dry seasons, that dams help control floods by releasing storm water gradually, and that farms use released water to irrigate crops.
  2. Answer: Flowing water spins a turbine connected to a generator.
    Water released from the reservoir flows through turbines, spinning their blades; each turbine connects to a generator, which turns that spinning motion into electricity, as described in the passage's second paragraph.
  3. Answer: About 2,080 megawatts.
    The passage states that Hoover Dam's power plant was built to generate up to about 2,080 megawatts of electricity. That is its capacity; the passage also notes that drought has cut the power it actually makes by about 30 percent.
  4. Answer: A steeper drop gives water more force at the turbine.
    The passage explains that electricity output depends on both flow and head, the distance water drops before reaching the turbines. A larger drop means each gallon of water arrives at the turbine with more force, so a steep-river dam can generate more power than a flat-river dam even with a similar flow.
  5. Answer: Any one of: blocked fish passage, changed habitat, or sediment buildup.
    Any one of: blocking fish such as salmon from reaching spawning waters upstream, changing the habitat from a flowing river to still reservoir water, or sediment settling in the reservoir and slowly reducing its storage capacity.
  6. Answer: Answers will vary but should reference that the passage ties power output to two separate factors, flow and head, meaning a dam with a large flow but little elevation drop, or a large drop but little flow, would not generate as much electricity as a dam strong in both factors together.

Differentiation

Support (below grade level)

Provide a simple labeled diagram of a dam cross-section (reservoir, turbine, generator, downstream river) for students to reference while reading, and allow question 2 to be answered by pointing to and labeling the diagram instead of writing a full explanation.

Extension (above grade level)

Ask students to explain, using only the passage's information, why a multi-year drought that lowered Lake Mead's water level significantly would cut Hoover Dam's power output, and to support their reasoning with the passage's flow-and-head explanation.

ELL supports

Pre-teach "reservoir," "turbine," "generator," and "head" with a simple diagram showing each part in sequence, labeled in English and, where possible, in students' home languages. Allow questions 1, 3, and 5 to be answered with short phrases copied directly from the passage rather than full sentences.