Lesson plan Grade 6 Science

Elderwood Dam and the Fish Ladder: An Ecosystem Change Argument

A 50-minute grade 6 lesson where students use a fictional dam's fish-count data to calculate percent change and argue how much a fish ladder restored the population.

Grade: Grade 6
Subject: Science
Material: Lesson plan
Time: 50 min
Prep: Low prep
Materials: Fish-count data handout, Sentence-frame handout for argument paragraphs, Index cards

Standards

MS-LS2-4 NGSS

Construct an argument supported by empirical evidence that changes to physical or biological components of an ecosystem affect populations.

How this material aligns: Students construct a written argument from the before-dam, after-dam, and after-ladder fish-count data to explain how a physical change to Elderwood Creek affected the fish population, the empirical-evidence argument MS-LS2-4 requires.

All materials for MS-LS2-4 →

WHST.6-8.1 Common Core ELA

Write arguments focused on discipline-specific content.

How this material aligns: The independent-practice paragraph requires a stated claim, at least two pieces of numeric evidence from the fish-count data, and a sentence of reasoning, the discipline-specific argument structure WHST.6-8.1 requires in a science context.

All materials for WHST.6-8.1 →

6.RP.A.3 Common Core Math

Use ratio and rate reasoning to solve real-world and mathematical problems, e.g., by reasoning about tables of equivalent ratios, tape diagrams, double number line diagrams, or equations, including finding a percent of a quantity and converting measurement units.

How this material aligns: Guided practice has students calculate the percent decrease in fish counts after the dam was built and the percent increase after the fish ladder was installed, applying the percent-of-a-quantity reasoning 6.RP.A.3 targets.

All materials for 6.RP.A.3 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-16.

Objectives

Students will calculate percent change in a fish population before and after physical changes to a stream.
Students will construct a written argument, supported by data, about whether a fish ladder restored a fish population.

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.

Also: spawning, population, claim, evidence

Teacher overview

Elderwood Dam, a fictional structure built decades ago on Elderwood Creek to store water, blocked adult fish from swimming upstream to the gravel beds where they had always spawned. This lesson gives students a simple three-point fish-count record, before the dam, after the dam but before any fix, and after a fish ladder was added, and asks them to calculate percent change and then argue how complete the recovery really was. A common misconception this lesson corrects directly is that installing a fish ladder "fixes" a blocked stream entirely; the data students calculate show a real but partial recovery, which is a more accurate and more common outcome than a full return to the original population.

Procedure

Hook (5 min): Ask students, "What do you think happens to a fish population if a wall suddenly blocks the river they use to swim upstream every year?" Collect two or three predictions on the board without confirming or correcting them.

Direct instruction (10 min): Introduce Elderwood Creek and Elderwood Dam, a fictional stream and dam used for this lesson. Explain that before the dam, biologists counted about 100 adult fish returning to spawn each year; after the dam was built, blocking upstream passage, that number fell to about 20 fish per year; years later, after a fish ladder, a stepped channel that lets fish swim around the dam, was installed, the count rose to about 65 fish per year. Frame this as a direct example of MS-LS2-4: a physical change to an ecosystem (the dam, then the ladder) changing a population (the fish).

Guided practice (15 min): In pairs, students calculate two values from the data: the percent decrease in fish counts from before the dam to after the dam, and the percent increase in fish counts from after the dam to after the fish ladder. Circulate to confirm pairs are using the correct starting value in each percent calculation, since using 100 as the base for both steps is the most common error. Introduce sentence frames for the next step: "The data show ___, which is evidence that ___."

Independent practice (15 min): Each student writes a short argument paragraph answering: "Did the fish ladder fully restore Elderwood Creek's fish population to what it was before the dam?" The paragraph must include a clear claim, at least two numbers from today's data as evidence, and one sentence explaining how the evidence supports the claim.

Closure / exit ticket (5 min): On an index card, students answer: "Is 65 fish per year, the count after the ladder, closer to full recovery (100 fish) or closer to the disaster-era count (20 fish)? Support your answer with one number from today's data."

Elderwood Dam and the Fish Ladder: An Ecosystem Change Argument — Grade 6 Science

Name: ____________________________ Date: ______________

Student Handout

Elderwood Creek fish counts (a fictional stream and dam used for this lesson). Before the dam, biologists counted about 100 adult fish returning to spawn each year. After the dam was built, that count fell to about 20 fish per year. Years later, after the fish ladder was installed, the count rose to about 65 fish per year. Use these three counts for all four questions.

What was the percent decrease in the fish count from before the dam (100 fish per year) to after the dam (20 fish per year)? Give your answer as a percent.
What was the percent increase in the fish count from after the dam (20 fish per year) to after the fish ladder (65 fish per year)? Give your answer as a percent.
Write a short argument paragraph answering: "Did the fish ladder fully restore Elderwood Creek's fish population to what it was before the dam?" Include a clear claim, at least two numbers from the counts above as evidence, and one sentence explaining how the evidence supports the claim.
Exit ticket: Is 65 fish per year, the count after the ladder, closer to full recovery (100 fish) or closer to the disaster-era count (20 fish)? Support your answer with one number from the counts above.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

Answer: 80 percent.
Guided practice, percent decrease after the dam: 80 percent. (100 − 20) ÷ 100 × 100 = 80 ÷ 100 × 100 = 80% fewer fish returned after the dam was built.
Answer: 225 percent.
Guided practice, percent increase after the fish ladder: 225 percent. (65 − 20) ÷ 20 × 100 = 45 ÷ 20 × 100 = 225% more fish returned after the ladder, compared with the disaster-era count.
Answer: The fish ladder helped, but it did not fully restore the population.
Independent practice, model argument: A strong paragraph states a claim such as "The fish ladder helped, but it did not fully restore the population," cites that the count rose from 20 to 65 fish (a 225% increase) while still sitting well below the original 100 fish, and reasons that 65 fish represents only 65% of the original baseline (65 ÷ 100 × 100 = 65%), so real progress occurred without a complete recovery.
Answer: Closer to full recovery.
Exit ticket answer: 65 fish per year is closer to full recovery than to the disaster-era count: it is more than three times the 20-fish low point, though it still falls short of the original 100-fish baseline by 35 fish.

Differentiation

Support

Provide the two percent-change calculations in guided practice with the subtraction step already completed (80 and 45), so students only need to complete the division and multiplication steps, and offer the sentence frames as a fill-in-the-blank rather than an open paragraph for independent practice.

Extension

Ask students to calculate what fish count Elderwood Creek would need to reach for a full 100% recovery to the original baseline, expressed both as a raw number and as a percent increase over the current 65-fish count, and to discuss what stream conditions beyond the fish ladder might still be limiting the population.

ELL supports

Pre-teach "fish ladder," "spawning," "population," "claim," and "evidence" with a simple diagram of a dam, a stepped fish ladder alongside it, and fish swimming through the steps. Allow the independent-practice paragraph to be built from the sentence frames with numbers filled in, rather than written as free-form prose, for students who need that scaffold.

Extensions

Have students consider (using only the lesson's data and vocabulary, not outside sources) what other stream conditions besides a physical barrier, such as water temperature or dissolved oxygen, could keep a fish population below its original baseline even after a fish ladder restores physical passage, and share one hypothesis with the class.