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Lesson plan Grade 7 Science

Copperline Creek's Watershed: Measuring 20 Years of Paving Over

A grade 7 lesson calculating the percent increase in a fictional watershed's impervious surface over 20 years, linked to faster stormwater runoff.

Objectives

  1. Students will explain how impervious surfaces change the amount and speed of stormwater runoff in a watershed.
  2. Students will calculate the percent increase in a watershed's impervious surface coverage over time.
  3. Students will compare percent changes across two watersheds to reason about which has changed more.

Vocabulary

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.
Impervious surface
An impervious surface is any hard surface, like a road, roof, or parking lot, that rain cannot soak through. The more impervious surface a town has, the more stormwater rushes off it.

Also: runoff, runoff coefficient

Teacher overview

This lesson uses a fictional watershed, Copperline Creek, to connect a familiar sight, new roads, roofs, and parking lots, to a measurable change in how a watershed responds to rain. Impervious surfaces like pavement carry a runoff coefficient around 0.9, meaning roughly 90 percent of the rain that falls on them runs off immediately, compared to a coefficient around 0.2 for lawns and other vegetated ground, where most rain soaks in instead. A common misconception is that a small rise in the percent of paved area, from 18 percent to 33 percent for instance, sounds like a modest change; the percent-increase calculation in this lesson shows students that same change is closer to an 83 percent jump in paved coverage, a very different picture.

Procedure

Hook (5 min): Describe two aerial views of the same neighborhood 20 years apart: the first with scattered houses and open fields, the second with more roads, driveways, and rooftops filling in the same area. Ask students what they notice has changed and how that might affect a nearby creek during a heavy rain.

Direct instruction (12 min): Define watershed and impervious surface, and present the real runoff-coefficient contrast: pavement runs off at a coefficient around 0.9, while lawns run off at a coefficient around 0.2, meaning pavement sends more than four times as much of a given rainfall directly into storm drains and creeks instead of letting it soak into the ground. Introduce Copperline Creek's fictional watershed data: 20 years ago, 18 percent of the watershed was impervious surface; today, after new development, 33 percent of the watershed is impervious surface.

Guided practice (15 min): In pairs, students use Percent increase = (new − old) ÷ old × 100 to answer two questions about Copperline Creek's watershed, then discuss how a faster, larger volume of runoff during storms could affect flash flooding along the creek.

  1. What is the percent increase in Copperline Creek's impervious coverage from 18 percent to 33 percent?
  2. What percent of the watershed remains pervious (unpaved) today, versus 20 years ago?

Independent practice (8 min): Each student answers one question, then writes one sentence comparing the result to Copperline Creek's.

  1. A smaller, second watershed nearby went from 10 percent to 25 percent impervious surface over the same 20 years. What is its percent increase?

Closure / exit ticket (5 min): Students answer one question on an index card.

  1. A third watershed went from 40 percent to 44 percent impervious surface. Is that a bigger or smaller percent increase than Copperline Creek's change from 18 percent to 33 percent? Show your work. Collect and scan for the correct percent-increase calculation 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: 83 percent.
    Guided practice, question 1: (33 − 18) ÷ 18 × 100 = 15 ÷ 18 × 100 ≈ 83% increase in impervious surface over 20 years.
  2. Answer: 67 percent.
    Guided practice, question 2: pervious (unpaved) land dropped from 82 percent (100 − 18) 20 years ago to 67 percent (100 − 33) today, over the same 20-year period.
  3. Answer: 150 percent.
    Independent practice: (25 − 10) ÷ 10 × 100 = 15 ÷ 10 × 100 = 150% increase for the second watershed, a larger percent increase than Copperline Creek's 83% even though the raw starting and ending percentages are smaller.
  4. Answer: smaller.
    Exit ticket: (44 − 40) ÷ 40 × 100 = 4 ÷ 40 × 100 = 10% increase, much smaller than Copperline Creek's roughly 83% increase, even though both changes represent real new development.

Differentiation

Support (below grade level)

Provide the percent-increase formula as a labeled arrow rule ("new minus old, divided by old, times 100") on a printed strip, and complete Copperline Creek's calculation together as a class before pairs attempt the second watershed independently. Allow a hundredths grid to shade in the "before" and "after" impervious percentages side by side as a visual check.

Extension (above grade level)

Ask students to calculate what percent impervious surface Copperline Creek's watershed would need to reach for its percent increase (from the original 18 percent) to hit 100 percent, and to explain what that milestone would mean in plain language (the watershed's paved area would have doubled from its starting point).

ELL supports

Pre-teach "watershed," "impervious," and "runoff" with a simple side-by-side diagram: rain hitting pavement with arrows showing water running off quickly, and rain hitting grass with arrows showing water soaking in. Allow the independent-practice comparison sentence to be spoken aloud to a partner instead of written.