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Reading passage Grade 6 Science

What Lives Under Wrenmere Creek's Gravel Bed

A grade 6 science reading passage on the water that moves through a creek's gravel bed, with fictional monitoring data and six comprehension questions.

Objectives

  1. Students will describe the path water takes through the gravel under a streambed.
  2. Students will explain how bacteria in the gravel remove nitrate from creek water.
  3. Students will use monitoring data to compare a clean reach with a silted reach.

Vocabulary

Nitrate
Nitrate is a form of nitrogen that plants use as food. It gets into water from fertilizer, animal waste, and septic systems, and too much of it is unhealthy for both people and streams.
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.

Also: riffle, hyporheic zone, silt, larvae

Teacher overview

This passage introduces the hyporheic zone: the water moving slowly through the gravel below the streambed, and the bacteria and insect larvae that live down there. It pairs the description with a short monitoring record from two reaches of the same fictional creek, one with clean gravel and one filled with silt, so that students can see what happens when that underground flow is blocked. Teachers can assign it before a stream field trip or alongside a unit on water quality. One misconception worth naming first: students commonly judge a stream by how clear the water looks, and clear water above a silted bed is exactly the case this passage examines.

What Lives Under Wrenmere Creek's Gravel Bed — Grade 6 Science
Name: ____________________________ Date: ______________

Passage

Under the Gravel of Wrenmere Creek

Wrenmere Creek runs past the edge of Ombury, a small fictional town. Everything in this passage about Ombury and Wrenmere Creek was written for this reading.

On a cool morning in October, a sixth grade class met a stream crew at the footbridge. The students expected to study the water they could see. The crew started them somewhere else. One of the crew explained that a good deal of a creek moves through the gravel under the streambed, out of sight, and that the class would have to dig to reach it.

Look closely at a shallow, rocky stretch of a creek. The water there runs fast over loose stones. That kind of stretch is called a riffle. Where a riffle begins, the bed slopes gently down. Some of the creek water presses into the spaces between the stones and keeps going underground. It moves slowly through those spaces, then rises back into the creek at the lower end of the riffle. The layer of wet gravel where this happens has a name. Scientists call it the hyporheic zone. A drop of water may spend minutes down there, or hours.

Plenty lives in that gravel. A thin, slippery film of bacteria coats each stone. Insect larvae crawl through the openings between the stones. Young stoneflies and mayflies are common there. A larva is the early life stage of an insect that will later fly. These living things need oxygen, and the water sinking in from above carries it to them.

The bacteria also do something useful for the creek. Leaves and bits of plants wash down and settle into the gravel. Bacteria break that material apart and use oxygen as they work. Deeper in, where the oxygen has run low, a different group of bacteria keeps going by using nitrate instead. Nitrate is a nutrient. Too much of it feeds heavy algae growth downstream. When these bacteria use nitrate, they change most of it into nitrogen gas, and that gas leaves the water for the air. Nitrate that goes into the gravel does not always come back out.

The crew took the class to two stretches of Wrenmere Creek. The first was above the old quarry road, where the gravel is loose and clean. The second was a quarter mile below it. A building site there had washed fine silt into the creek all spring, and the silt had settled into the spaces between the stones.

At each stretch the crew measured the nitrate in the water entering and leaving. They pushed a probe about 20 centimeters into the gravel to measure dissolved oxygen. Then they scooped one sample of gravel at each site and counted the stonefly larvae in it.

Table 1. Wrenmere Creek monitoring record (fictional crew data)

Measurement Clean gravel stretch Silted stretch
Nitrate entering the stretch 2.8 mg/L 2.9 mg/L
Nitrate leaving the stretch 1.6 mg/L 2.8 mg/L
Dissolved oxygen 20 cm into the gravel 6.4 mg/L 1.1 mg/L
Stonefly larvae counted in one sample 34 3
Silt filling the spaces between stones Very little Heavy

The water in both stretches looked the same from the bridge. It was clear, and it moved at about the same speed. A neighbor walking his dog stopped and said the creek had never looked better.

The crew wrote the numbers on a clipboard and said little. Back in the classroom, they asked the students to compare the two columns and decide whether the neighbor was right. They also said the town has one decision to make before spring: whether to require the builder to install silt fencing before the next rainy season.

Questions

  1. According to the passage, what happens to some of the creek water where a riffle begins?
  2. The passage describes two things the bacteria in the gravel use as they break down leaves and plant bits. Name both, and say which one the bacteria use when the other has run low.
  3. Compare the nitrate entering and leaving each stretch in Table 1. What does the comparison show about where the creek removes nitrate?
  4. Only three stonefly larvae turned up in the silted stretch, compared with 34 in the clean stretch. Use the dissolved oxygen readings to explain why.
  5. The neighbor judged the creek by looking at it from the bridge. Use two details from the passage or the table to explain why clear water was not enough to judge the silted stretch.
  6. The town must decide whether to require silt fencing at the building site. Using evidence from the passage, explain what silt fencing would protect that a person standing on the bridge cannot see.

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: It presses down into the spaces between the stones and travels underground through the gravel. The passage explains that the bed slopes down where a riffle begins, and that the water rises back into the creek at the lower end of the riffle.
  2. Answer: Oxygen and nitrate; the bacteria use nitrate deeper down, where the oxygen has run low. The passage states that a different group of bacteria keeps working by using nitrate instead of oxygen.
  3. Answer: The creek removes nitrate in the clean gravel stretch and almost none in the silted stretch. Nitrate dropped from 2.8 mg/L to 1.6 mg/L in the clean gravel, while in the silted stretch it went from 2.9 mg/L to 2.8 mg/L, which is nearly unchanged.
  4. Answer: The gravel in the silted stretch held very little oxygen, so few larvae could live there. The probe read 6.4 mg/L of dissolved oxygen in the clean gravel and only 1.1 mg/L in the silted gravel, and the passage says the larvae depend on oxygen carried down by water sinking into the stones.
  5. Answer: Clear water tells you nothing about the spaces between the stones. Accept any two supporting details, such as the silt filling those spaces, the low oxygen reading in the silted gravel, the small number of larvae, or the nitrate that passed straight through.
  6. Answer: It would protect the open spaces in the gravel, and the living things and nitrate removal that depend on water moving through them. The passage explains that silt from the building site settled between the stones, and the table shows that where the silt is heavy, the oxygen, the larvae, and the nitrate removal all drop.

Differentiation

Support

Read the passage aloud while students follow, pausing after the paragraph about nitrate to draw a simple cross section on the board: creek water on top, an arrow pushing down into gravel, and an arrow rising back up. Give these students the table with the two oxygen readings highlighted. Assign questions 1 through 4, and provide sentence frames such as "The nitrate went from ___ to ___ in the ___ stretch, which shows ___."

Extension

Ask students to write the paragraph the stream crew would add to its report recommending, or declining to recommend, the silt fencing, using at least three readings from the table. A second option is to design the next round of sampling: name one more stretch to test, say why that stretch was chosen, and predict the oxygen reading.

ELL supports

Pre-teach "riffle," "gravel," "silt," "larvae," and "nitrate" with photographs or drawings and with two clear jars, one of clean gravel and one of gravel packed with mud, that students can pour water through. Provide a bilingual glossary card. Allow oral answers for questions 1 and 2, and accept a labeled drawing as the answer to question 5.

Extensions

Have students find a photograph of a stream riffle from a state agency or university site and mark where water would enter and leave the gravel. A second option is to find out whether their own county requires erosion control at building sites, and to report what a permit asks a builder to install before ground is broken.