Flowing Waters

Background

Hydrologists calculate flow on rivers and streams to understand how much water is available for human consumption, agriculture, livestock, industry, and for wildlife habitat. Traditionally, stream flow is calculated as the unit "cfs" (cubic feet per second).

Procedure

Divide the class into groups of four. Each group will calculate flow along a different (but nearby) 30-foot reach of creek. They will compare results. Since they will be measuring flow along reaches that are nearby, their flow calculations should be the identical.

Because Medano Creek has surge flow and because the water level is relatively shallow, it may be difficult to obtain precise measurements. But by comparing results among groups, students will be able to critically discuss the reasons for the varied measurements and apply the scientific method to their studies. The most accurate measurements will be taken along the more constricted, deeper areas of the creekbed. A wide, braided section would be difficult to measure accurately.

Step 1: Estimate Velocity

Groups will use time measurements to calculate the velocity of the creek. They must first measure out a 30-foot section of the creek that is as straight as possible. Place markers with colored ribbon where everyone can see them to define the beginning and the end of each section (reach) of creek. Have one student at the beginning of each section drop a floating object. A partner at the end of the reach will time how long it takes for the floating object to get to the end marker. The recorder will write down the time on the worksheet. The fourth student can catch the floating object and bring it back to the beginning. Since flow rate varies across the width of a stream and due to the surge flow, students should take four time measurements and average the results. (See worksheet for details.)

Step 2: Estimate Area

Each group should now estimate the area of a cross-section within their reach. Choose a cross-section halfway between the two markers. First, have two students measure the width of the creek. The measuring can be done with a tape measure or less accurately with a yardstick. Have the recorder write down the measurement in feet on the worksheet.

Now two students should measure the depth of the creek in three equally-spaced locations along the cross-section. Use the yardsticks to locate the bottom and then read the measurement (in inches). The recorder will write down the measurements. The worksheet has a work area which will allow students to convert the measurements to feet and calculate the average depth.

Multiply the width times average depth to get an estimated cross-section area.

Step 3: Calculate Flow

Once all groups have finished, gather together to share information and to calculate water flow for each group's section of creek. To do this, use the formula on the worksheet: velocity times area equals flow.

Compare results.

Critical Thinking

1. Why are there differences in the groups' calculations?
2. Could the flow be different in the different sections of creek?
3. What natural processes could have affected measurements?
4. Where were the measurements most accurate?
5. What steps could have created errors?
6. How could the process be modified to reduce the potential for errors?

Extension

Older students can practice calculating stream flow using Go with the Flow.

Adapted from Educator's Guide to Great Sand Dunes, by Lori Cooper, Friends of the Dunes.