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Worksheet Grade 6 Science

Sediment in Cardew Reservoir: Storage Lost Behind Thrushmoor Dam

A grade 6 worksheet on how sediment fills Cardew Reservoir behind Thrushmoor Dam, using percent, ratios, and acre-foot to gallon conversions.

Objectives

  1. Students will calculate what percent of a reservoir's original storage has been replaced by sediment.
  2. Students will convert a reservoir volume from acre-feet to gallons using a given conversion factor.
  3. Students will explain how weathering and runoff carry sediment from a watershed into a reservoir.

Vocabulary

Acre-foot
The amount of water it takes to cover one acre of land, about the size of a football field, one foot deep. Western states use acre-feet to measure large amounts of water.
Watershed
A watershed is all the land where rain that falls will drain to the same stream, river, or lake. Everyone lives in a watershed, no matter how far from the water they live.

Also: sediment, storage capacity, dredging

Teacher overview

Cardew Reservoir is the drinking water source for the fictional town of Thrushmoor, and it sits behind Thrushmoor Dam at the bottom of a steep watershed. This worksheet gives students a survey record for the reservoir and asks them to work out how much of its original storage has already been replaced by sediment. The misconception worth addressing directly is that a reservoir's capacity is permanent, because the dam and the shoreline never appear to move and students commonly assume the lake will always hold what it held on opening day. Every creek that fills a reservoir also carries rock and soil into it, so storage shrinks a little every year even when the dam itself is in good condition.

Sediment in Cardew Reservoir: Storage Lost Behind Thrushmoor Dam — Grade 6 Science
Name: ____________________________ Date: ______________

Student Worksheet

Cardew Reservoir, a fictional lake, was built behind Thrushmoor Dam. When the lake was new, it could hold 4,000 acre-feet of water. An acre-foot is enough water to cover one acre of ground one foot deep. A recent survey used sonar to map the lake bottom. The survey found that sediment now fills 600 acre-feet of the reservoir. Two creeks bring sediment down from the hills. The survey team estimates that about 12 acre-feet of new sediment arrives each year. The town of Thrushmoor draws 850 acre-feet from the reservoir each year. One acre-foot holds 325,851 gallons. Use these numbers to answer the questions.

  1. The reservoir was built to hold 4,000 acre-feet. Sediment now fills 600 acre-feet. What percent of the original storage is sediment?
  2. The reservoir was built to hold 4,000 acre-feet, and 600 acre-feet are now sediment. How many acre-feet of water storage are left?
  3. One acre-foot holds 325,851 gallons. Change 3,400 acre-feet into gallons.
  4. Sediment arrives at about 12 acre-feet each year. How many years will it take for another 120 acre-feet of sediment to build up?
  5. The town of Thrushmoor uses 850 acre-feet each year. If 3,400 acre-feet of storage are left, how many years of town use is that?
  6. There are 600 acre-feet of sediment today. Sediment keeps arriving at 12 acre-feet each year. How many acre-feet of sediment will there be after 25 more years?
  7. Suppose the sediment total reaches 900 acre-feet. What percent of the reservoir's original 4,000 acre-feet is that?
  8. Write the ratio of the sediment today, 600 acre-feet, to the original storage, 4,000 acre-feet. Put the ratio in simplest form.
  9. Rain and freezing weather break rock and soil loose on the hills above the lake. Creeks then carry that material downhill. Explain in two or three sentences how this process turns hillside rock into reservoir sediment.
  10. One creek drains a steep slope where a fire burned off the plants. The other creek drains a flat meadow covered in thick grass. Which creek most likely carries more sediment into Cardew Reservoir? Explain your choice.

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: 15 percent.
    Divide the sediment by the original storage, so 600 divided by 4,000 gives 0.15. Multiply by 100 to get 15 percent.
  2. Answer: 3,400 acre-feet.
    Subtract the sediment from the original storage: 4,000 - 600 = 3,400 acre-feet.
  3. Answer: 1,107,893,400 gallons.
    Multiply the remaining storage by the conversion factor: 3,400 x 325,851 = 1,107,893,400 gallons. Students who set the work up as 325,851 x 34 and then multiply by 100 reach the same total.
  4. Answer: 10 years.
    Divide the added sediment by the yearly rate: 120 divided by 12 is 10 years.
  5. Answer: 4 years.
    Divide the remaining storage by the yearly draw: 3,400 divided by 850 is 4 years. Check by multiplying, since 850 x 4 = 3,400 acre-feet.
  6. Answer: 900 acre-feet.
    Twenty-five years of sediment is 12 x 25 = 300 acre-feet. Add the sediment already there: 600 + 300 = 900 acre-feet.
  7. Answer: 22.5 percent.
    Divide the sediment total by the original storage, so 900 divided by 4,000 gives 0.225. Multiply by 100 to get 22.5 percent.
  8. Answer: 3 to 20.
    Start with 600 to 4,000 and divide both numbers by 200. The simplest form is 3 to 20. Accept the fraction three twentieths written in place of the ratio.
  9. Answer: Weathering loosens rock, and moving water carries the pieces into the lake.
    Look for three ideas: rain and freezing break rock into smaller pieces, runoff and creek water pick those pieces up and move them downhill, and the water slows when it reaches the reservoir so the pieces settle to the bottom. A response that names only one of the three earns partial credit.
  10. Answer: The creek draining the steep burned slope.
    Plant roots hold soil in place, so the grassy meadow releases far less material. A steep slope also moves water faster, and faster water carries larger and heavier pieces. Accept any answer that names the burned slope and gives either the missing plant cover or the steeper grade as the reason.

Differentiation

Support

Give students a partially completed conversion frame for item 3 with the multiplication already set up as 3,400 x 325,851, and allow calculators throughout. Before the worksheet, walk the class through one percent problem with friendlier numbers, such as 200 acre-feet of sediment in a lake built to hold 1,000 acre-feet, so the method is established before the town of Thrushmoor numbers appear.

Extension

Ask students to find how many more years it would take for sediment to fill half of Cardew Reservoir if the rate stays at 12 acre-feet each year, then write a short recommendation to the town of Thrushmoor about whether dredging the lake or planting the burned slope would be the better response.

ELL supports

Pre-teach "sediment," "acre-foot," and "storage capacity" with a labeled cross-section sketch of the reservoir showing the water layer above and the sediment layer below. Provide sentence frames for items 9 and 10, such as "Rain and ice break the rock into ___, and the creek carries it to ___," and "The creek from the ___ slope carries more sediment because ___."

Extensions

Have students compare Cardew Reservoir with a lake or pond near your school by tracing the land that drains into it on a topographic map. Students can then predict which of the two would collect sediment faster and explain the prediction with the same reasoning they used for the two creeks above the town of Thrushmoor.