Water for Schools
Menu

Lesson plan Grade 8 Math

Pebble Run Storm Hydrograph: Reading and Sketching Graphs

A 45-minute grade 8 lesson where students treat a creek's storm hydrograph as a function of time and sketch and describe its rise, peak, and slow fall after a rainstorm.

Objectives

  1. Students will explain why streamflow after a storm is a function of time.
  2. Students will sketch and describe a hydrograph's rising limb, peak, and recession.

Vocabulary

Key words: hydrograph, function, peak, recession, lag

Teacher overview

After a rainstorm, the flow in a creek does not jump and drop with the rain; it rises with a delay, reaches a single peak, and then falls slowly for hours or days. A graph of that flow over time is called a hydrograph, and it is a clear example of a function because each moment has exactly one flow value. This lesson uses the fictional Pebble Run creek so students can read the shape of a hydrograph, connect it to the definition of a function, and sketch one from a verbal description. Because the work is about the shape of the graph rather than exact numbers, students record their reasoning in words and sketches.

Procedure

Hook (5 min): Describe Pebble Run on a dry week: a low, steady trickle. Then ask what students think happens to that flow during and after a heavy afternoon storm. Collect predictions about when the creek is highest.

Direct instruction (12 min): Define the parts of a hydrograph on a projected or drawn axis with time across the bottom and streamflow up the side: the low steady baseflow before the storm, the rising limb, the single peak, and the long recession afterward. Point out that the peak comes some time after the rain starts, because water needs time to run off the land and reach the creek. State the function idea: at each time there is exactly one flow, so streamflow is a function of time.

Guided practice (12 min): Using a described hydrograph on the Student Handout, work items 1 through 4 together, naming each part of the shape. Pause after item 2 for a check for understanding: ask for a thumbs up or down on the statement "the creek peaks at the same instant the rain starts." Look for thumbs down, and have a student explain the lag before moving on.

Independent practice (11 min): Students complete items 5 and 6 on their own, deciding whether streamflow is a function of time and sketching a full hydrograph for the described storm on a blank grid.

Closure and exit ticket (5 min): Students answer one question aloud or on a card: "After the peak, is the flow rising or falling, and does it get back to normal quickly or slowly?" Listen for "falling slowly," which confirms they understand the recession.

Pebble Run Storm Hydrograph: Reading and Sketching Graphs — Grade 8 Math
Name: ____________________________ Date: ______________

Student Handout

Pebble Run is a fictional creek. Before a storm it flows low and steady. One afternoon, heavy rain falls for about two hours. Use that story for the questions below.

  1. Before the rain, the creek flows low and steady. What is this part of a hydrograph called?
  2. Does the flow jump up at the exact moment the rain starts, or a bit later? Explain why in one sentence.
  3. The part of the graph where the flow climbs is called the rising limb. Sketch what that part looks like.
  4. After the peak, the rain has stopped and the flow slowly drops. What is this falling part called?
  5. Is streamflow a function of time here? Use the idea that each input has exactly one output to explain.
  6. Sketch a full hydrograph for this storm: low baseflow, a delayed rise, a single peak, and then a slow fall back toward baseflow.

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: Baseflow. It is the low, steady flow the creek has before the storm adds any runoff.
  2. Answer: A bit later. Rain must first run off the land and travel to the creek, so the flow rises after a lag rather than the instant the rain begins.
  3. Answer: A curve that climbs from the low baseflow up toward the peak, getting steeper as more runoff arrives.
  4. Answer: The recession, or recession limb. It is the long, gentle fall in flow after the peak as the runoff drains away.
  5. Answer: Yes. At each moment in time the creek has exactly one flow value, so streamflow is a function of time; a single time cannot have two different flows.
  6. Answer: A graph that starts low and flat (baseflow), rises after a short delay along the rising limb, reaches one peak, then falls slowly along the recession back toward the baseflow level.

Differentiation

Support

Provide a partly drawn hydrograph with the baseflow and rising limb already sketched so students add only the peak and recession. Give the four part-names on a small word bank next to the grid.

Extension

Ask students to sketch a second hydrograph on the same axes for a storm with the same total rain falling over six hours instead of two, and explain why its peak is lower and later.

ELL supports

Pre-teach "baseflow," "rising limb," "peak," and "recession" with a labeled picture of a full hydrograph. Pair each term with a hand motion (flat, up, top, slow down) that the class performs together while naming the parts.