Lesson 9: Building an Evidence-Based Explanation

Download Lesson 9 Professional Development
“This is the lesson where the rubber meets the road.” Mr. Showalter, PA teacher

This is the moment when everything starts to click. After weeks of investigations, data collection, and system modeling, students return to the unit driving question:

How can we balance winter safety with the health of our watershed?

Students understand why salt is used and have seen its safety benefits. They have also gathered evidence that road salt does not simply disappear after it is spread. It dissolves, moves with melting snow and stormwater, and can enter local streams and groundwater. Their data reveal conductivity spikes after winter storms, measurable chloride in local streams, and shifts in freshwater organisms that point to a deeper question: What story does our evidence tell about road salt in our watershed?

In this lesson, students synthesize evidence from across the unit to build an explanation of what is happening, how they know, and what questions still remain. They determine whether the evidence suggests a healthy system, a salt-related concern, or a more complex picture that requires deeper investigation.

This is sensemaking. Students move from collecting data to constructing and defending explanations. They look for patterns over time, trace cause-and-effect relationships, and connect land use, runoff pathways, water chemistry, and ecosystem health into a coherent account of their watershed. Rather than viewing each investigation in isolation, they integrate their learning to explain how winter road salt moves through a connected system and why it matters.

This synthesis marks the turning point of the unit. Before proposing solutions, students must first be confident in their explanation of the problem. By grounding their conclusions in evidence, they are prepared to thoughtfully consider how communities can protect winter safety while also safeguarding freshwater ecosystems.

In this lesson, students will dive into their data. It might feel messy at first—but that’s exactly what real science looks like. Like professional scientists, students will sift through complex, sometimes conflicting evidence to tell a story about their local watershed and the impact of road salt. By exploring their own data with purpose, students sharpen their ability to reason through evidence and draw conclusions about real-world environmental challenges.

Teacher Reference: Evidence Students Will Synthesize

Students will organize evidence from four categories developed throughout the unit.

1. National Evidence
Lesson 1: National data showing increasing road salt use and rising freshwater salinity

2. Movement Through the Watershed
Lesson 2: Watershed mapping and River Runner pathways showing how water flows through connected systems
Lesson 4: Schoolyard runoff investigations identifying impervious surfaces and infiltration patterns
Lesson 5: Erosion table investigations and runoff simulations modeling how land use influences water movement and pollutant transport

3. Our Stream Chemistry
Lesson 6: Storm event conductivity data showing spikes following winter salting events
Lesson 7: Local stream chemistry investigations measuring chloride and conductivity

4. Biological Evidence
Lesson 8: Macroinvertebrate investigations providing biological evidence of long-term stream health

Materials:

Materials you will need to gather:

Learning Objectives:

At the completion of this lesson, students will be able to:

  • Combine information from various investigations to draw conclusions. STEELS Standard(s): 3.4.6-8.E, 3.5.6-8.O
  • Formulate a claim supported by evidence. STEELS Standard(s): 3.4.6-8.E
  • Describe the importance of skepticism in scientific inquiry (Nature of Science). STEELS Standard(s): 3.5.6-8.O, 3.5.6-8.H
  • Engage in collaborative scientific work (Nature of Science).

Activities:

This is the moment when students bring the entire unit together. Over the past several weeks, students have gathered many different kinds of evidence about how winter road salt moves through a watershed. Now they step back and ask one important scientific question: What story does all of our evidence tell about how winter road salt moves through our watershed and affects stream health?

Remind students that the goal is not to find a single “right” answer. The goal is to develop the strongest explanation possible using the evidence they have collected. You might say: “We’ve collected a lot of evidence. Today, your job is to decide what story that evidence tells. Does the evidence suggest that winter road salt is moving through our watershed and affecting stream health? What evidence supports your thinking?”

Teacher Tip: Before teaching this lesson, update the presentation by replacing the example slides with your class’s data from the Movement Through the Watershed, Stream Chemistry, and Biological Evidence investigations.

Examining the Evidence

Distribute the Data Synthesis Organizer. Display each evidence slide one at a time (1. National Evidence, 2. Movement Through the Watershed, 3. Our Stream Chemistry, and 4. Biological Evidence), allowing students time to discuss each piece of evidence with their group before recording their thinking. As students complete the organizer, encourage them to look for patterns across multiple investigations rather than focusing on individual data sets. Ask questions such as:

  • What does this evidence suggest?
  • How does it connect to evidence from previous investigations?
  • Does this evidence support or complicate what we already know?

Remind students that scientists rarely rely on a single investigation. Instead, they compare many different types of evidence to develop explanations about how complex systems work.

Looking Across the Evidence

Once students have completed all four sections of the organizer, ask them to step back and look across all of the evidence.

  • What patterns do you notice?
  • Which pieces of evidence support one another?
  • Did any evidence surprise you or raise new questions?

Emphasize that no single investigation tells the whole story. Strong scientific explanations are built by connecting many different types of evidence.

Constructing a Claim

Working in groups, students use the final section of the organizer to develop one evidence-based claim. Provide the sentence frame:

Since __________________ and also __________________, __________________… we think __________________.

Encourage students to use evidence from at least two different investigations and to explain how those pieces of evidence connect. Remind students that scientific conclusions often include uncertainty—phrases such as the evidence suggests…, may be contributing…, and based on the evidence we collected… are appropriate when supported by data.

Whole-Class Synthesis

Invite each group to share its claim and supporting evidence. As groups present, record their claims where the class can compare similarities and differences. Guide the discussion by asking:

  • Which pieces of evidence appeared in many groups’ claims?
  • Did any groups interpret the evidence differently?
  • Which explanations were supported by the strongest evidence?
  • What additional evidence would increase our confidence?

Conclude by reminding students that this is exactly how environmental scientists investigate complex problems. Scientists gather many different types of evidence, discuss competing explanations, and refine their conclusions as new evidence becomes available. The claims students develop today will provide the scientific foundation for Lesson 10, when they design solutions that balance winter safety with protecting freshwater ecosystems.

Adaptations & Extensions:

Extension 1: Data Story

Assign individual students to create a one-page visual “data story” that combines graphs, photos, and a narrative explanation of what the evidence reveals about their local watershed.

Assessment:

Assessment in this lesson is embedded in the activity itself. Students demonstrate understanding by completing the Data Synthesis Organizer, developing an evidence-based claim using the sentence frame, and participating in the Whole-Class Synthesis discussion. Strong responses will draw on evidence from at least two different investigations and connect that evidence to an explanation of how road salt moves through the watershed and affects stream health.