Worksheet Grade 7 Science

Bramford Brook's Storm Hydrograph: Peak Flow and Lag Time

A grade 7 science worksheet reading an hourly storm hydrograph from a fictional brook: baseflow, peak, lag time, percent recession, flood stage, and warning time.

Grade: Grade 7
Subject: Science
Material: Worksheet
Time: 40 min
Prep: No prep
Materials: Printed worksheet, Calculators, Graph paper

Standards

MS-ESS3-2 NGSS

Analyze and interpret data on natural hazards to forecast future catastrophic events and inform the development of technologies to mitigate their effects.

How this material aligns: Students interpret gauge data from one storm to identify flood conditions and estimate the warning time a downstream village would receive in a future storm.

All materials for MS-ESS3-2 →

7.RP.A.3 Common Core Math

Use proportional relationships to solve multistep ratio and percent problems. Examples: simple interest, tax, markups and markdowns, gratuities and commissions, fees, percent increase and decrease, percent error.

How this material aligns: Items 5 and 6 require ratio and percent work on the discharge record, item 6 being a multistep percent decrease across the storm's recession.

All materials for 7.RP.A.3 →

7.RP.A.2 Common Core Math

Recognize and represent proportional relationships between quantities, including identifying the constant of proportionality and representing proportional relationships by equations.

How this material aligns: Item 8 treats peak discharge as proportional to rainfall depth and scales the first storm's peak to predict a second one, and item 9 asks why that proportional model falls short.

All materials for 7.RP.A.2 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-09-04.

Objectives

Students will read baseflow, peak discharge, and time of peak from an hourly gauge record.
Students will calculate lag time between the middle of the rainfall and the peak discharge.
Students will use the record to estimate flood warning time for a downstream village.

Vocabulary

Baseflow: Baseflow is the water that keeps a stream flowing even when it has not rained for a while. It comes from groundwater slowly seeping into the streambed.
Flood stage: Flood stage is the water level in a river at which it starts to overflow its banks and cause flooding problems nearby. Forecasters compare the current water level to flood stage to warn people when flooding is about to happen.

Also: hydrograph, discharge, peak flow, lag time, recession

Teacher overview

A stream gauge reports a storm as a curve rather than as a single number, and reading that curve is how flood forecasters decide when to warn a village downstream. This worksheet gives students one storm from a gauge on a fictional brook and asks for the quantities forecasters actually pull off it: baseflow, peak discharge, lag time, recession, and warning time. A common misconception is that the peak arrives while the rain is still falling; the lag-time item is placed early so the record itself corrects that.

Bramford Brook's Storm Hydrograph: Peak Flow and Lag Time — Grade 7 Science

Name: ____________________________ Date: ______________

Student Worksheet

Bramford, Bramford Brook, and Rothley Bridge are fictional, and all of the readings below were written for this exercise. The gauge at Rothley Bridge reports discharge in cubic feet per second, written cfs. Rain fell steadily from 3:00 p.m. to 5:00 p.m. and totaled 1.6 inches.

Time	Discharge (cfs)
2:00 p.m.	15
3:00 p.m.	15
4:00 p.m.	45
5:00 p.m.	125
6:00 p.m.	140
7:00 p.m.	130
8:00 p.m.	95
9:00 p.m.	60
10:00 p.m.	40
11:00 p.m.	25
Midnight	18

Baseflow is the flow in the brook before the storm runoff arrives. What was the baseflow at the 2:00 p.m. and 3:00 p.m. readings?
What was the highest discharge in the record?
At what time did that highest discharge occur?
The rain fell from 3:00 p.m. to 5:00 p.m., so the middle of the rainfall was 4:00 p.m. Lag time is the time from the middle of the rainfall to the peak. What was the lag time?
The peak discharge was 140 cfs and the baseflow was 15 cfs. How many times greater was the peak than the baseflow? Round to the nearest tenth.
Discharge fell from 140 cfs at 6:00 p.m. to 60 cfs at 9:00 p.m. What was the percent decrease? Round to the nearest whole percent.
Bramford Brook spills over its banks when discharge rises above 120 cfs, the flow at which the water reaches flood stage. How many of the hourly readings in the table are above 120 cfs?
A second storm drops 3.2 inches of rain, twice the 1.6 inches of the first storm. If peak discharge were proportional to rainfall, what peak would you predict?
The real peak from that second storm was higher than a proportional prediction. Give one reason the ground itself could cause that.
Suppose Bramford paves a large part of the land that drains to the brook. Explain what would happen to the lag time and to the peak discharge in the next storm.
The village of Bramford sits 6 miles downstream of Rothley Bridge. High water travels that stretch at 3 miles per hour. How many hours of warning does a peak at the bridge give the village?

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: 15 cfs.
Both the 2:00 p.m. and the 3:00 p.m. readings show 15 cfs, so that steady value is the baseflow before storm runoff reaches the gauge.
Answer: 140 cfs.
Scanning the discharge column, 140 cfs is the largest value in the record.
Answer: 6:00 p.m.
The 140 cfs reading is recorded at 6:00 p.m.
Answer: 2 hours.
The middle of the rainfall was 4:00 p.m. and the peak came at 6:00 p.m., so the lag time is 6:00 minus 4:00, or 2 hours.
Answer: 9.3 times.
140 cfs divided by 15 cfs equals about 9.33, which rounds to 9.3 times the baseflow.
Answer: 57 percent.
The drop is 140 minus 60, or 80 cfs. Dividing 80 by the starting value of 140 gives about 0.571, which rounds to a 57 percent decrease.
Answer: 3 readings.
The readings above 120 cfs are 125 cfs at 5:00 p.m., 140 cfs at 6:00 p.m., and 130 cfs at 7:00 p.m.
Answer: 280 cfs.
Doubling the rainfall from 1.6 inches to 3.2 inches doubles the predicted peak, and 140 cfs times 2 equals 280 cfs.
Answer: The ground was already wet from the first storm.
Soil that is already saturated cannot soak up much more water, so a larger share of the second storm runs off the surface instead of infiltrating, which pushes the peak above a simple proportional estimate. Accept any answer naming wet, saturated, or frozen ground as the reason less water soaks in.
Answer: Lag time gets shorter and peak discharge gets higher.
Pavement does not let water soak in, so runoff reaches the brook faster and in greater volume. The peak arrives sooner after the rain and reaches a higher discharge than it did before the paving. Accept any answer stating both a shorter lag and a higher peak.
Answer: 2 hours.
6 miles divided by 3 miles per hour equals 2 hours between the peak at Rothley Bridge and the arrival of that high water at the village.

Differentiation

Support

Give students a version of the table with the baseflow rows and the peak row already highlighted, and a printed number line of clock hours so item 4 can be counted rather than subtracted. Provide one worked percent-decrease example using values that are not on this worksheet.

Extension

Have students plot the eleven readings on graph paper with time on the horizontal axis, draw a smooth curve, and mark the rising limb, the peak, and the recession. Ask them to sketch a second curve showing how the same storm would look after the watershed is paved.

ELL supports

Pre-teach "discharge," "peak," "lag," and "flood stage" using a labeled sketch of the curve with an arrow for each term. Provide sentence frames for items 9 and 10: "The peak was higher because the ground was ___." and "After paving, the lag time would be ___ and the peak would be ___."