Last Wednesday our River Classroom headed north to the Rio Chama to fulfill a data request. Several interested parties wanted to know the speed of the river below Abiquiu Dam.
There are several ways to measure the velocity of a river. The professionals use current meters, which can cost hundreds of dollars. Right now that’s a little out of our budget. We decided to use oranges instead.
How, you may ask, can we measure river velocity with oranges?
Using math, of course!
First, you measure out a distance. We used 20 meters. Station one student at the top of this stretch with an orange. This student will hold the orange in the river, and when he or she releases it, they yell “time”.
At this point, another student on the shore of the river starts a stopwatch and tracks the orange down the river.
At the end of the section of river, several students should be waiting with nets to catch the floating orange. This is not as easy as it sounds, so students should be staggered in case one misses!
When the orange crosses the finish line, the student in charge of the stopwatch stops the timer and reports the time to the official data recorder. So now we have a distance and the time it took an orange to float that distance. If only there were some equation that we could use to determine speed. But wait! There is!
distance = rate x time
rate = distance / time
With this clever use of math, our students found the speed of the Rio Chama at different intervals from the bank! They were proud of their accomplishment.
As students were leaving for the day, we noticed an official looking truck down river at the USGS stream gauge. It turned out to be an official scientist from the USGS who was measuring stream discharge AND stream velocity! While we weren’t able to hang around to watch the process, he did share his results with us. He found the velocity of the Rio Chama to be between 0.3 and 0.6 m/s, which perfectly matches our data!
Traditionally, oranges are used to measure river velocity. We weren’t sure why, so we decided to test apples and grapefruit as well. The numbers were closer than we expected. The apple isn’t the best choice because it doesn’t float evenly. It hits the river bottom and spins in shallow water.
This activity was really fun, and the students enjoyed the calculations involved. We hope to repeat this again later in the year when the river is at different flow levels so that we can correlate river discharge and river velocity.