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Reading passage Grade 7 English Language Arts

Water That Powers a City: Hydropower at Cassin Dam

A grade 7 passage on how a hydroelectric dam turns falling water into electricity, with comprehension questions that mix literal recall and inference.

Objectives

  1. Students will explain the sequence by which a hydroelectric dam turns falling water into electricity.
  2. Students will cite evidence to support an inference about a tradeoff of building a dam.

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, tradeoff

Teacher overview

This informational passage explains how a hydroelectric dam turns the simple fact of falling water into electricity for homes. It uses the fictional Cassin Dam as its main example and names the real Hoover Dam to show the scale large dams can reach. The passage also raises a fair tradeoff, so students can practice both tracing a process and weighing costs and benefits with textual evidence.

Water That Powers a City: Hydropower at Cassin Dam — Grade 7 English Language Arts
Name: ____________________________ Date: ______________

Passage

Every time you turn on a light, the electricity may have started as water sitting quietly behind a dam. Turning falling water into power is called hydropower, and it is one of the oldest ways people have made electricity. The fictional Cassin Dam, on the edge of a mid-sized city, shows how the process works.

A dam is a strong wall built across a river. Behind the wall, the river backs up into a large body of stored water called a reservoir. The key to hydropower is that the water in the reservoir sits high above the river on the far side of the dam. Water that is held up high has stored energy, simply because gravity will pull it down if it gets the chance.

At Cassin Dam, that chance comes through a large pipe built into the dam. When the operators open a gate, water rushes down the pipe from the reservoir. At the bottom, the fast-moving water strikes the blades of a turbine, which is a wheel built to spin when water pushes on it. The falling water spins the turbine quickly and steadily.

The spinning turbine is connected to a generator. Inside the generator, the spinning motion is turned into electricity. From there, wires carry the electricity out of the dam and into the city's power lines. As long as water keeps falling through the pipe, the turbine keeps spinning and the generator keeps making power.

Two things decide how much electricity a dam can make. The first is how far the water falls. A taller dam drops its water from a greater height, so the water arrives at the turbine with more force. The second is how much water flows through. More water striking the turbine each second means more power. A dam with both a tall drop and a heavy flow can make a great deal of electricity.

The size range is wide. Cassin Dam is a medium dam that helps power one city. The real Hoover Dam, on the border between Nevada and Arizona, is far larger. Its power plant is rated to produce up to about 2,080 megawatts, enough to help supply cities across several states. Years of drought have lowered Lake Mead, the reservoir behind Hoover Dam, and cut the dam's power output by about 30 percent. Both dams work on the same idea; they differ mostly in how tall they are and how much water moves through them.

Dams do more than make power. The reservoir behind a dam stores water that a region can use during dry months. A dam can also hold back a surge of water during a heavy storm and release it slowly, which helps protect towns downstream from flooding. For a dry or flood-prone region, these uses can matter as much as the electricity.

Still, a dam changes the river it crosses, and that is a real tradeoff. Fish that once swam freely up and down the river may be blocked by the wall. The stretch of river below the dam receives less water at certain times, which can change the plants and animals that live there. Engineers try to reduce these effects, for example by building fish ladders that let fish climb past the dam. But a dam is a large change to a natural system, and communities weigh its benefits against its costs before they build one.

Hydropower shows how a basic idea, water falling because of gravity, can be shaped by careful engineering into steady electricity for a whole city.

Questions

  1. What is hydropower, according to the passage?
  2. What is a reservoir, and where is it located in relation to the dam?
  3. In order, what happens after operators open the gate at Cassin Dam and water rushes down the pipe?
  4. According to the passage, what two things decide how much electricity a dam can make?
  5. How does the passage use Hoover Dam to make a point about dam size? Cite evidence in your answer.
  6. Besides making electricity, what two other uses of a dam does the passage describe?
  7. Explain one tradeoff of building a dam. Cite at least two details from the passage that support your answer.
  8. Based on the passage, why might a community in a dry, flood-prone region especially want a dam, even beyond the electricity?

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: Hydropower is turning falling water into electricity. The passage defines it as "turning falling water into power."
  2. Answer: A reservoir is the large body of stored water that backs up behind the dam, sitting high above the river on the far side. The passage states this directly.
  3. Answer: The water strikes the turbine's blades and spins it; the spinning turbine turns a connected generator; the generator makes electricity, which wires carry to the city. This is the ordered chain in the passage.
  4. Answer: How far the water falls and how much water flows through. The passage names both the height of the drop and the amount of flow.
  5. Answer: It contrasts a medium dam with a very large one. The passage says Hoover Dam "is rated to produce up to about 2,080 megawatts," far more than Cassin Dam, to show how wide the size range is while both use the same idea.
  6. Answer: Storing water for dry months and holding back storm water to protect towns from flooding. Both uses are stated in the passage.
  7. Answer: Answers should cite that a dam can block fish that "once swam freely" and that the river below "receives less water at certain times," changing the plants and animals there. Two such details support the tradeoff.
  8. Answer: Answers will vary but should infer that the reservoir stores water for dry months and the dam controls floods, so both water supply and flood protection would matter greatly in such a region.

Differentiation

Support

Provide a labeled diagram of a dam (reservoir, pipe, turbine, generator, power lines) for students to reference, and let question 3 be answered by numbering steps on the diagram before writing. Allow phrase-length answers for questions 1, 2, and 4.

Extension

Ask students to write a short paragraph arguing whether a town in the passage's dry, flood-prone region should build a dam, using at least one benefit and the tradeoff, supported by evidence from the text.

ELL supports

Pre-teach "reservoir," "turbine," "generator," and "tradeoff" with pictures and simple definitions before reading. Provide a sentence frame for the evidence questions: "The passage says ___, which shows ___."