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Reading passage Grade 7 Science

What Gantry Dam Changed Downstream: A Grade 7 River Reading

A grade 7 science reading passage on how a dam changes the river below it, from blocked fish migration and lower oxygen to trapped sediment and a steadier flow.

Objectives

  1. Students will identify four ways a dam changes the river below it.
  2. Students will explain how lower dissolved oxygen affects fish downstream.
  3. Students will cite details from the text to support an inference about populations.

Vocabulary

Dam
A strong wall built across a river to hold back water. Dams store water for people and farms, help prevent floods, and can make electricity.
Reservoir
A human-made lake, usually created by a dam, that stores water for later. Reservoirs supply cities and farms and help control floods.
Dissolved oxygen
Dissolved oxygen is the oxygen gas mixed into water that fish and other underwater animals breathe. Cold, moving water holds more oxygen than warm, still water.
Fish ladder
A series of water steps built beside a dam so migrating fish, like salmon, can swim past it. Fish leap or swim from pool to pool to reach the river above.

Also: migration, sediment, ecosystem

Teacher overview

A dam is one of the largest changes people can make to a river, and its effects reach far past the wall itself. This passage explains four ways a dam changes the river below it and how those changes affect the animals that live there. It works well as an introduction to how a physical change to an ecosystem can shift its populations, and it pairs with a diagram of a river above and below a dam.

What Gantry Dam Changed Downstream: A Grade 7 River Reading — Grade 7 Science
Name: ____________________________ Date: ______________

Passage

A dam is a wall built across a river. It holds back water to form a reservoir, and many dams also make electricity as water flows through them. A dam can be very useful. But it also changes the river below it, and those changes reach the animals that live there. Gantry Dam is an imagined example that shows how.

The first change is a barrier. Many fish swim upstream each year to lay their eggs. This trip is called migration. A dam wall blocks that path. Fish that once reached the upper river can no longer get there. Over time, fewer young fish hatch, and the fish population below the dam can shrink. To help, engineers sometimes build a fish ladder. A fish ladder is a set of small pools that rise like steps, letting fish move up and around the dam.

The second change is the water itself. A reservoir is deep, and the water near the bottom is cold and low in oxygen. When a dam releases this deep water, the river below can suddenly turn cold. It can also carry less oxygen. Most fish need dissolved oxygen above 5 mg/L to stay healthy. If the released water drops below that level, fish are put under stress, and some may not survive.

The third change is sediment. A river carries sand, silt, and small bits of rock. This material is called sediment, and it builds the bars and beds that many animals use. Behind a dam, the water slows down and drops its sediment on the reservoir floor. The water that leaves the dam is clear, but it is also "hungry." Clear, fast water picks up new material from the riverbed, so it can wear away the banks and beds below the dam.

The fourth change is the flow. A natural river rises and falls with the seasons. Spring floods spread gravel and clean the streambed. A dam can hold back these floods to store water or make power. The flow below then becomes steadier and calmer. That may sound better, but many plants and animals depend on the natural rise and fall, so a steady flow can change which species live there.

None of this means a dam is only harmful. Dams store water for dry months, control floods, and make electricity without burning fuel. But the river below is an ecosystem, and each of these changes can shift the balance of that ecosystem. People who manage a dam can reduce some of the effects. They can spill water over gates so it mixes with air and gains oxygen. They can release water from different depths to keep the temperature closer to normal. They can time releases to act more like a natural flood. Studying a river above and below a dam helps people weigh its benefits against the changes it brings.

Questions

  1. According to the passage, what does a dam create that blocks migrating fish?
  2. What is a fish ladder, and what problem does it help solve?
  3. The passage says most fish need dissolved oxygen above a certain level. What is that level?
  4. Why might the water released from deep in a reservoir harm fish just below the dam? Use details from the passage.
  5. The passage says the water leaving a dam is often clear but "hungry." Explain how clear water can still damage the riverbanks.
  6. Using at least two details from the passage, explain why building a dam can change the populations of animals living downstream.

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: A barrier, the dam wall.
    The passage says the dam wall blocks the path that fish use to swim upstream to lay their eggs.
  2. Answer: A set of step-like pools that let fish move around the dam.
    The passage explains that a fish ladder is small pools that rise like steps, which helps solve the problem of fish being blocked from swimming upstream.
  3. Answer: Above 5 mg/L.
    The passage states that most fish need dissolved oxygen above 5 mg/L to stay healthy.
  4. Answer: The deep water can be cold and low in oxygen.
    The passage says water near the bottom of a reservoir is cold and low in oxygen, so releasing it can drop the river below 5 mg/L and put fish under stress.
  5. Answer: Clear water carries little sediment, so it picks up new material and erodes the banks.
    The passage explains that "hungry" water without sediment picks up material from the riverbed and can wear away the banks and beds below the dam.
  6. Answer: Answers will vary but should use at least two details.
    Accept any two of: the dam blocks fish migration so fewer young hatch; the released water can be cold and low in oxygen; the dam traps sediment and erodes downstream habitat; and the steadier flow changes which species can live there. Each of these changes can shift the populations of animals downstream.

Differentiation

Support

Provide a labeled diagram showing the reservoir, the dam wall, and the river below, with the four changes marked. Let students answer questions 1, 2, and 3 by pointing to the part of the passage that states each fact before writing.

Extension

Ask students to choose one of the four changes and describe one more way a dam operator could reduce it, using only the ideas in the last paragraph as a starting point.

ELL supports

Pre-teach "dam," "reservoir," "migration," and "sediment" with simple pictures. Allow students to answer the literal questions (1, 2, 3) with short phrases, and provide a sentence frame for question 6: "A dam changes animal populations because ___ and ___."