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Why a Lake Turns Over Twice a Year

8th grade nonfiction passage about lake layers and turnover: 278 words, six questions, an answer key and vocabulary.

8th grade nonfiction lake layers and turnover 278 words

Why a Lake Turns Over Twice a Year

In a deep lake in a cool climate, the water is not mixed evenly. For much of the year it separates into layers, and twice a year those layers trade places in an event called turnover. The key to the story is a peculiar property of water: it is densest at about four degrees Celsius, not at its freezing point.

In summer, the sun warms the top of the lake. Warm water is lighter than cold water, so it stays on the surface, floating above a deep layer of cold, dense water. Between them lies a narrow zone where the temperature drops quickly. Wind can stir the upper layer, but it cannot easily push through that barrier, so the bottom water is cut off from fresh air. In a lake rich in nutrients, the oxygen in the deep water may slowly run low.

As autumn arrives, the surface water cools and becomes heavier. Eventually it is as dense as the water beneath it, and nothing holds the layers apart. Wind then mixes the whole lake from top to bottom. This is the fall turnover. It carries oxygen down to the depths and lifts nutrients toward the light.

In winter, the surface may cool below four degrees, becoming lighter again, and ice forms on top. The lake settles into layers once more, this time with the coldest water at the surface. When spring warms the ice and the surface water, the process reverses. The top water sinks until the lake is mixed again, which is the spring turnover. These two mixings keep a lake supplied with oxygen, which is why fish and other animals can live in its depths.

Questions

  1. At about what temperature is fresh water the densest? in the text

  2. How many times a year does a lake like this turn over? in the text

  3. What barrier keeps summer wind from mixing the bottom water? in the text

  4. Why might the deep water of a summer lake run low on oxygen? think about it

  5. Why does fall turnover begin when the surface water cools? think about it

  6. Why does turnover matter to animals living deep in a lake? think about it

Words to know

turnover
the mixing of a lake's layers so top and bottom trade places
dense
heavy for its size, with its parts packed closely together
barrier
something that blocks the way or keeps things apart
nutrients
substances that living things need in order to grow
layers
separate levels lying one on top of another
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One passage, three levels

The same topic and facts written three ways, so a mixed class can read and talk about one text. Each group’s sheet shows only a shape; the key says which is which.

● Easier · 249 words · Flesch-Kincaid grade 3.5

A deep lake in a cool place is not mixed evenly. For much of the year it forms layers. Twice a year, the layers trade places. This is called turnover. It happens because of a strange fact about water. Water is densest at about four degrees Celsius, not at its freezing point.

In summer, the sun warms the top of the lake. Warm water is lighter than cold water, so it floats on top. Below it is a deep layer of cold, heavy water. Between them is a narrow zone where the temperature drops fast. Wind can stir the top layer, but it cannot push through this barrier. The bottom water gets no fresh air. In a lake with a lot of nutrients, the oxygen at the bottom may slowly run low.

In autumn, the top water cools and gets heavier. At last it is as dense as the water below. Nothing keeps the layers apart. Wind mixes the whole lake. This is the fall turnover. It carries oxygen down and lifts nutrients up toward the light.

In winter, the top water may cool below four degrees and get lighter again. Ice forms on top. The lake forms layers again, with the coldest water at the top. In spring, the sun warms the ice and the top water. The top water sinks until the lake is mixed again. This is the spring turnover. These two mixings give a lake oxygen, so fish and other animals can live deep down.

▲ This passage · 278 words · Flesch-Kincaid grade 6.7

In a deep lake in a cool climate, the water is not mixed evenly. For much of the year it separates into layers, and twice a year those layers trade places in an event called turnover. The key to the story is a peculiar property of water: it is densest at about four degrees Celsius, not at its freezing point.

In summer, the sun warms the top of the lake. Warm water is lighter than cold water, so it stays on the surface, floating above a deep layer of cold, dense water. Between them lies a narrow zone where the temperature drops quickly. Wind can stir the upper layer, but it cannot easily push through that barrier, so the bottom water is cut off from fresh air. In a lake rich in nutrients, the oxygen in the deep water may slowly run low.

As autumn arrives, the surface water cools and becomes heavier. Eventually it is as dense as the water beneath it, and nothing holds the layers apart. Wind then mixes the whole lake from top to bottom. This is the fall turnover. It carries oxygen down to the depths and lifts nutrients toward the light.

In winter, the surface may cool below four degrees, becoming lighter again, and ice forms on top. The lake settles into layers once more, this time with the coldest water at the surface. When spring warms the ice and the surface water, the process reverses. The top water sinks until the lake is mixed again, which is the spring turnover. These two mixings keep a lake supplied with oxygen, which is why fish and other animals can live in its depths.

■ Harder · 305 words · Flesch-Kincaid grade 8.3

In a deep lake located in a cool climate, the water is not mixed evenly throughout the year. For most of the year it separates into distinct layers, and twice annually those layers exchange places in an event called turnover. The key to understanding it is an unusual property of water: it is densest at roughly four degrees Celsius rather than at its freezing point.

During summer, the sun heats the top of the lake. Warm water is lighter than cold water, so it remains at the surface, floating above a deep layer of cold, dense water. Between the two layers lies a narrow zone in which the temperature falls sharply. Wind can stir the upper layer, but it cannot easily force its way through that barrier, so the bottom water becomes cut off from fresh air. In a lake that is rich in nutrients, the oxygen in the deep water may gradually run low.

When autumn arrives, the surface water cools and becomes heavier. Eventually it reaches the same density as the water beneath it, and nothing remains to hold the layers apart. Wind is then able to mix the entire lake from top to bottom. This is the fall turnover, and it carries oxygen down to the depths while lifting nutrients toward the light.

During winter, the surface may cool below four degrees, becoming lighter once again, and ice forms across the top. The lake settles into layers a second time, now with the coldest water at the surface. When spring warms the ice and the surface water, the process runs in reverse. The top water sinks until the lake is thoroughly mixed again, which is the spring turnover. These two mixings keep a lake supplied with oxygen, which is the reason fish and other animals are able to live in its depths.

Download as one PDF

How to use it

  1. Press Read it big (or B) to put the text on the projector, or Full screen (F) to fill the whole screen.
  2. Ask the questions one at a time and use Show answer when the class has decided. A shows or hides every answer.
  3. Print student copy for the class, or Print with answers for yourself.

Ideas for class

  • Before reading, put turnover, dense, barrier, nutrients and layers on the board with their meanings from the Words to know section and have students predict what the text will be about.
  • Read it aloud once while students follow along, then have them reread it alone or with a partner. At a calm 100 words a minute it takes about 3 minutes to read aloud.
  • Use the Read it big button (or B) to put the text on the projector, and Show answer on one question at a time so the class commits to an answer first.
  • Question 4 asks the reader to infer. Ask students to point to the words in the passage that led them to their answer.
  • Ask students to write one question the passage did not answer, then look for the answer in a book or on a classroom screen.
  • Print the student copy for independent work and keep the answer-key copy for yourself.

Related

Questions

What reading level is "Why a Lake Turns Over Twice a Year"?

It was written for 8th grade. It has 278 words and works out to a Flesch-Kincaid grade level of about 6.7. That is a formula's estimate from sentence length and syllables, and it is not a Lexile or a guided reading level, so check it against your students.

What do the questions ask?

There are six. 3 ask for something the text says directly and 3 ask the reader to work something out from clues. Each has an answer in the key, and the answers are written as a teacher would accept them, not as the only possible wording.

How long does it take to read?

About 3 minutes read aloud at 100 words a minute. For a one-minute fluency check, paste it into the fluency timer and use the word count.

Can I print it?

Yes. Print student copy gives the passage, the questions with lines to write on, and the word meanings. Print with answers gives the same page with the answers filled in. Both are made in your browser.

Is this passage from a published book?

No. It was written for Peabody Teach, so there is no book to buy and no login to read it.

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