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Lesson plan Grade 9 Science

Kettle Bend Dam: Weighing Fish Passage Options With Cost-Benefit Math

A grade 9 ecology lesson where students evaluate three options for a fictional dam blocking trout habitat, calculating cost per mile of habitat regained for each.

Objectives

  1. Students will explain how a dam can block fish migration and reduce accessible spawning habitat.
  2. Students will calculate cost per mile of habitat regained for competing dam-management options.
  3. Students will recommend and justify a solution using both ecological and cost-benefit evidence.

Vocabulary

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.
Hydropower
Electricity made from moving water, usually water falling through a dam. It is a renewable power source that can start up quickly when the electric grid needs it.

Also: fish passage, spawning habitat, cost-benefit ratio

Teacher overview

The town of Kettle Bend sits beside a dam built in 1932 on the Anders River (a fictional river used for this lesson) that generates a small amount of hydropower and once provided flood control for downstream farmland. The dam also completely blocks native trout from reaching about 20 miles of spawning habitat upstream that the population used before the dam was built. The town council is weighing three options: leave the dam as it is, retrofit it with a fish ladder, or remove the dam entirely. This lesson has students evaluate the three options using both ecological reasoning and cost-benefit math.

A common misconception this lesson addresses is that a fish ladder fully solves a dam's fish-passage problem, restoring habitat access to something close to pre-dam conditions. In practice, fish ladders only let a fraction of migrating fish pass through successfully, so a "solved" fish-passage problem on paper can still leave most of the historic habitat effectively out of reach. The math in this lesson is built to surface that gap directly.

Procedure

Hook (5 min): Display or sketch a diagram of a dam with fish attempting to swim upstream and being blocked by the dam wall. Ask students what they think happens to a fish population over many years if it can no longer reach the habitat where it has always spawned. Take a few predictions without confirming or correcting yet.

Direct instruction (15 min): Remind students that Kettle Bend Dam and the Anders River are fictional, and that every dollar and mile figure in this lesson belongs to that invented scenario. Explain that dams change river ecosystems in several ways: they block fish migration, change water temperature and flow patterns, and can trap sediment that would otherwise rebuild downstream habitat. Introduce Kettle Bend's three options: Option A, no action (keep the dam exactly as it is); Option B, retrofit the dam with a fish ladder; and Option C, remove the dam entirely. Present the option data: Option A costs nothing but restores 0 miles of habitat and keeps the dam's 2 megawatts (MW) of hydropower generation. Option B costs an estimated $1.2 million and, because ladders are imperfect, engineers estimate it would let fish pass successfully to only 40 percent of the historic 20 miles of habitat, while keeping the 2 MW of hydropower. Option C costs an estimated $3.5 million (removal and streambed restoration) and would reopen all 20 miles of habitat, but it would end the dam's 2 MW of hydropower generation entirely.

Guided practice (15 min): As a class, calculate how many miles of habitat Option B's fish ladder would realistically make accessible: 20 miles × 0.40 = 8 miles. Discuss what the 8 miles represents. The ladder does not block 12 of the miles outright; on average about 4 of every 10 fish that try the ladder get through, so the spawning benefit is roughly equal to fully reopening 8 of the 20 miles.

Independent practice (15 min): Students work individually or in pairs on the numbered items below to complete the cost-benefit comparison.

  1. Using the guided-practice result, how many miles of the 20-mile historic habitat would Option B's fish ladder make accessible?
  2. Calculate Option B's cost per mile of habitat regained: $1.2 million ÷ your answer to item 1.
  3. Calculate Option C's cost per mile of habitat regained: $3.5 million ÷ 20 miles.
  4. Based on items 2 and 3 alone, which option costs less per mile of habitat regained, B or C?
  5. Option B's lower cost per mile does not automatically make it the better choice. Name one factor besides cost per mile that the town council should weigh before deciding, and explain why it matters.

Closure / exit ticket (5 min): Students answer individually on an index card: "Which of the three options would you recommend to the Kettle Bend town council, and what is the strongest piece of evidence, ecological or financial, supporting your choice?" Collect the cards and scan for a clear recommendation backed by at least one specific number or ecological fact from the lesson before dismissal.

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: 8 miles.
    20 miles × 0.40 (the fish ladder's estimated passage rate) = 8 miles of habitat made accessible.

  2. Answer: $150,000.
    $1,200,000 ÷ 8 miles = $150,000 per mile of habitat regained.

  3. Answer: $175,000.
    $3,500,000 ÷ 20 miles = $175,000 per mile of habitat regained.

  4. Answer: Option B.
    Option B's cost per mile ($150,000) is lower than Option C's ($175,000), even though Option C regains more total miles of habitat.

  5. Answer: short explanation.
    A reasonable answer names the loss of the dam's 2 MW of hydropower generation under Option C (a factor Option B avoids), or notes that Option C restores full access to all 20 miles rather than a partial 8 miles, meaning the two options are not delivering an equivalent ecological outcome even though B looks cheaper per mile.

Exit ticket answer: Answers will vary. A strong response names one specific option and supports it with either the cost-per-mile figures (items 2 and 3) or the ecological point that a fish ladder passes only a fraction of the population while full removal restores complete access, rather than a general statement with no number or fact attached.

Differentiation

Support (below grade level)

Provide a printed table with columns for Option, Cost, Miles Regained, and Cost per Mile, with the cost and miles-regained columns already filled in, so students only need to complete the division for items 2 and 3.

Extension (above grade level)

Ask students to calculate, using only the fictional Kettle Bend lesson's own numbers and not outside sources, what percentage increase in fish ladder passage rate, above 40 percent, would be needed for Option B's cost per mile to drop below $100,000, and to show the calculation that supports their answer.

ELL supports

Pre-teach "spawning habitat," "fish ladder," and "cost-benefit" using the hook diagram, labeling the dam, the blocked fish, and the upstream habitat directly on the picture in English and students' home languages where possible. Allow item 5 and the exit ticket to be answered with a short phrase plus one supporting number instead of a full paragraph.