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Not Too Slow, Not Too Fast
Students will establish a distance on a stream bank, using a fixed-length rope and time the movement of a “rubber ducky” toy or block of wood in the stream along that distance. They will then calculate stream velocity and discuss the implications of the stream velocity to the quality of salmon habitat.
Essential Question(s)
PDF Investigation
Time Required
20 minutes
Life Sciences
Investigation Type
Field Trip
Grade Level
NGSS Performance Expectations
Materials Needed
  • 1 rubber ducky toy (A block of wood can be substituted.)
  • 1 Timer
  • 1 50’ measuring tape (a measured length of rope - 10 or 20 feet long – can be substituted)
  • Laminated sign that lists the velocities that young and adult coho salmon can swim against
    A young coho salmon can swim against a current of 1 foot/second.
    An adult salmon can swim against a current of 10 feet/second.
  • Laminated sign with the formula V = D/T, Velocity = Distance/Time
    (At least one student/group should have waterproof boots that won’t be overtopped by the stream.)

NGSS Performance Expectation

Students who demonstrate understanding can:

3-LS4-3. Construct an argument with evidence that in a particular habitat some organisms can survive well, some survive less well, and some cannot survive at all.


Knowledge - Students will know that:
Skills - Students will be able to:
Local and Cultural Connections
Teacher Preparation

Prepare science notebooks with a Velocity Datasheet.

Find a stream location where the stream bank is low and level and students can spread out along it without trampling vegetation. Choose the length you would like students to measure (e.g., 10 feet, 20 feet, 25 feet) based on the bank and channel characteristics. Avoid stream stretches where the rubber ducky might run into rocks or snags.

Ideally, you need at least 6 students to take measurements, one of whom has waterproof boots that won’t be overtopped by the stream. If you have additional students in your group, they can rotate the roles. The activity can also be done with only three students.

Learning Experiences


  1. Ask the students to tell you the question they are trying to answer: Could salmon survive well in this stream? Why or why not?
  2. Tell the students you will be collecting data about the velocity, or rate, at which the stream is flowing to provide evidence about whether or salmon can meet their habitat needs here.
  3. You can provide an example of velocity, or a rate of movement, by using “miles/hour.” Guide them into defining miles as a measure of “distance” and hour as a measure of “time.” So if a car has a velocity of 60 miles/hour, then the number 60 is the velocity which is a ratio of distance traveled (the numerator of a fraction) divided by the amount of time the car travels (the denominator of the fraction). So the velocity of your car is the same if you travel 120 miles in 2 hours or 60 miles in 1 hour.
  4. Hold up the sign that displays the formula V = D/T and tell them the units they will be using will be feet instead of miles and seconds instead of hours. Show them the distance you have chosen on the tape measure and stretch it out along the stream bank.


Set-up with six students:

  1. Select 6 students to participate in the measurements including at least one who is wearing boots that won’t be overtopped by standing in the water.
  2. Have two students standing on the bank hold the ends of the tape measure at the selected distance apart parallel to the stream so that it is taut.
  3. Position a third student on the bank at the upstream end of the tape measure (direction the water is flowing from) He or she will hold and then drop the rubber ducky.
  4. Position a fourth student on the bank at the downstream end of the tape measure (direction where the water is flowing to). He or she will have the timer.
  5. The fifth student is the only person who is required to be in the water, so should be wearing boots. He or she will catch the rubber ducky after it passes the end of the tape measure so that it can be used again.
  6. The sixth student will be the recorder of the data.

(With only three students, one would drop the rubber ducky while holding one end of the string, rope, or the measuring tape; another would catch it while holding the other end, and a third one would be the timer and recorder.)


  1. The students will time how long it takes the rubber ducky from one end of the tape measure to the other by having the person holding the rubber ducky drop it into the water at the same time the timer starts, and stopping the timer once the rubber ducky reaches the end of the measured distance.
  2. They will repeat the activity three times. The recorder will write down the record the time it takes for the rubber ducky to drift from the beginning to the end of the measured distance on paper for each of the three trials in his or her science notebook.
  3. When the three measurements are completed, get the group back together and have all of them record the three times in their science notebook and calculate the average of the three times.
  4. Remind them they already know the “D” in the velocity formula (the length of the string) and now they know the “T” in terms of the average time it took for the block of wood to travel that distance.

    V= D/T

    Velocity = Distance / Time

  5. Have them solve the formula for the velocity and write that in their science notebook. Share their answers until everyone agrees on the correct answer.


  1. Show students the sign that provides criteria for velocities that young coho salmon and adult coho salmon can swim against (1 foot/second and 10 feet/second, respectively) and ask them to write these down in the analysis section for this activity on their datasheet or in their science notebook.
  2. Discuss how calculating the velocity of the stream can tell us anything about whether salmon can live here during different parts of their life cycle:
    1. What might be missing that salmon and other living things need if a stream is moving very slowly? (oxygen)
    2. Which stages of salmon need oxygen? (all of them, including the eggs and alevins)
    3. What if the water is moving very fast, like during a flood? (Eggs and alevins could be
      washed out of the gravel and die; juvenile salmon and other fish would have to work hard to stay in their habitat, fighting the current and using energy instead of growing and they could be washed downstream and out to sea before they’re ready to live in salt water.)
    4. What adaptations might young salmon might have that allows them to live where there are high velocities or floods (staying behind logs and rocks or near the bottom or under banks, and then darting out into faster water to find prey).

Ask students to tell you why salmon need water that is “not too slow and not too fast.”

Draw their attention to the question on their data sheet or in their science notebook:

Based on your results, could salmon survive well in our stream? Yes or No & Why?

If you have time, have a discussion about how they would answer this question. Ask students to explain their answers by providing evidence to answer the “Why?” If there isn’t enough time for this discussion during the field trip, tell them to write their answers when they’re back in their classroom.

Velocity Datasheet  
Teacher Background

This field trip activity was developed for the Anchorage School District Watershed Education Program. The field trip program supplements a 4th grade STEM Kit on the theme of Interdependence and a focus on Anchorage watersheds and salmon.

Significance of Water Velocity for Salmon
Salmon require stream habitats where water velocities that provide enough turbulence to mix oxygen from the atmosphere into the water column but which are not too swift or turbulent too require fry and juveniles to expend energy to avoid being washed downstream from areas where they can find food. Coho salmon juveniles who spend up to three years in streams require velocities less than 1 foot/second. Different species of adult salmon vary in terms of their swimming abilities although some like cohos who move far up streams to spawn can swim against currents of 10 feet/second.
Measuring water velocities is a relatively simple sampling exercise, requiring only an object that can be observed and timed as it’s carried by the current. The accuracy of the measurement is limited, however, by taking the measurement at or near the surface of the stream since water velocities vary considerably throughout the water column and smaller fish and other animals are adapted to seek shelter from high velocities in a variety of ways.

Possible Learner Misconception(s) and Instructional Clarifications:

Learning Misconception: If the velocities are too high in the main channel of a stream, young salmon will be washed downstream.

Instructional Clarification: Salmon can live in stream stretches with high velocities through their adaptation of staying behind logs and rocks or near the bottom or under banks, and then darting out into faster water to find prey.

Learner Misconception: The macroinvertebrates that salmon depend on for food can’t survive in stream stretches with strong currents.

Instructional Clarification: Not all animals are as vulnerable to high velocities as juvenile salmon and salmon eggs and alevins. Some macroinvertebrate species (for example, black fly larvae) have adaptations to attach themselves to the bottom of rocks.

Components of Next Generation Science Standards Addressed

Science & Engineering Practices

Engaging in Argument from Evidence
Construct an argument with evidence.

Disciplinary Core Ideas

LS4.C: Adaptation
For any particular environment, some kinds of organisms survive well, some survive less well, and some cannot survive at all.

Cross-Cutting Concepts

Cause and Effect
Cause and effect relationships are routinely identified and used to explain change.

Common Core

RI.3.3 Describe the relationship between a series of historical events, scientific ideas or concepts, or steps in technical procedures in a text, using language that pertains to time, sequence, and cause/effect. (3-LS4-3)
MP.2 Reason abstractly and quantitatively. (3-LS4-3)

Also: 4.MD.A.2 Use the four operations to solve word problems involving distances, intervals of time, liquid volumes, masses of objects, and money, including problems involving simple fractions . . .
Alaska Cultural Standards
B. Culturally-responsive educators use the local environment and community resources on a regular basis to link what they are teaching to the everyday lives of the students.
Developed by Marilyn Sigman, Alaska Sea Grant
Last Updated on
Last Updated by
Marilyn Sigman
Alaska Sea Grant University of Alaska Fairbanks Alaska Department of Education and Early Development NOAA