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Why Lakes Flip Over Twice a Year

6th grade nonfiction passage about seasonal mixing in lakes: 205 words, six questions, an answer key and vocabulary.

6th grade nonfiction seasonal mixing in lakes 205 words

Why Lakes Flip Over Twice a Year

In many parts of the world, the water in a deep lake turns over twice a year. Nothing is flipped by hand. The mixing happens because water changes weight as it warms and cools.

In summer, the sun heats the top of a lake. Warm water is lighter, so it floats in a layer above the cold water below. The two layers hardly mix, and the deep water can slowly run low on oxygen.

In autumn, cool air and strong winds chill the surface. Cooler water is heavier and sinks, pushing deeper water upward. Soon the whole lake is stirred together. This is called turnover. It carries oxygen down to the bottom and lifts nutrients, the food that tiny plants need, up toward the light.

Winter brings a surprise. Water is heaviest at about 4 degrees Celsius, not at the freezing point. So the coldest water floats at the top and turns into ice, while slightly warmer water stays at the bottom. That is why fish can survive beneath a frozen lake.

In spring the ice melts, and the surface water warms until it reaches that heaviest point. It sinks, and the lake mixes a second time. Then summer returns, and the layers form once more.

Questions

  1. Why does warm water float above cold water in a summer lake? in the text

  2. What is turnover? in the text

  3. What does autumn turnover carry to the bottom of the lake? in the text

  4. At about what temperature is water heaviest? in the text

  5. Why can fish survive beneath a frozen lake in winter? think about it

  6. What causes the second turnover in spring? in the text

Words to know

turnover
the mixing of a lake from top to bottom
layers
levels that sit one above another
oxygen
the gas that animals need to breathe
nutrients
substances that living things need to grow
freezing
so cold that water turns to ice
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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 · 187 words · Flesch-Kincaid grade 2.8

In many parts of the world, the water in a deep lake turns over twice a year. No one flips it by hand. The mixing happens because water changes weight when it warms and cools.

In summer, the sun heats the top of the lake. Warm water is light, so it floats above the cold water. The two layers hardly mix. The deep water can run low on oxygen.

In autumn, cool air and wind chill the top of the lake. Cool water is heavy and sinks. It pushes deeper water up. Soon the whole lake is mixed. This is called turnover. It carries oxygen down to the bottom. It lifts nutrients, the food that tiny plants need, up to the light.

Winter brings a surprise. Water is heaviest at about 4 degrees Celsius, not at the freezing point. So the coldest water floats on top and turns to ice. Slightly warmer water stays at the bottom. That is why fish can live under a frozen lake.

In spring, the ice melts. The top water warms to the heaviest point and sinks. The lake mixes a second time.

▲ This passage · 205 words · Flesch-Kincaid grade 4.7

In many parts of the world, the water in a deep lake turns over twice a year. Nothing is flipped by hand. The mixing happens because water changes weight as it warms and cools.

In summer, the sun heats the top of a lake. Warm water is lighter, so it floats in a layer above the cold water below. The two layers hardly mix, and the deep water can slowly run low on oxygen.

In autumn, cool air and strong winds chill the surface. Cooler water is heavier and sinks, pushing deeper water upward. Soon the whole lake is stirred together. This is called turnover. It carries oxygen down to the bottom and lifts nutrients, the food that tiny plants need, up toward the light.

Winter brings a surprise. Water is heaviest at about 4 degrees Celsius, not at the freezing point. So the coldest water floats at the top and turns into ice, while slightly warmer water stays at the bottom. That is why fish can survive beneath a frozen lake.

In spring the ice melts, and the surface water warms until it reaches that heaviest point. It sinks, and the lake mixes a second time. Then summer returns, and the layers form once more.

■ Harder · 221 words · Flesch-Kincaid grade 6.2

In many regions of the world, the water in a deep lake turns over twice each year. Nothing is flipped by hand, of course. The mixing occurs because water changes in weight as it warms and cools.

During summer, the sun heats the upper part of a lake. Warm water is lighter, so it floats in a layer above the colder water below. The two layers scarcely mix, and the deep water can gradually run low on oxygen.

In autumn, cool air and strong winds chill the surface. The cooler water becomes heavier and sinks, pushing deeper water upward, and before long the entire lake is stirred together. This event is known as turnover. It carries oxygen down to the bottom and lifts nutrients, the food that microscopic plants need, up toward the sunlight.

Winter brings a surprising twist. Water is heaviest at about 4 degrees Celsius rather than at the freezing point. Consequently, the very coldest water floats near the top and turns into ice, while slightly warmer water remains at the bottom. That is the reason fish are able to survive beneath a frozen lake.

In spring, the ice melts, and the surface water warms until it reaches that heaviest point. It then sinks, and the lake mixes a second time. Summer soon returns, and the layers form once more.

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, layers, oxygen, nutrients and freezing 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 2 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 5 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 Lakes Flip Over Twice a Year"?

It was written for 6th grade. It has 205 words and works out to a Flesch-Kincaid grade level of about 4.7. 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. 5 ask for something the text says directly and 1 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 2 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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