A few times each semester, I take my students into the field to learn about data collection and apply the principles they’ve learned in the classroom. This week, my students are learning about rivers, so we headed out to the San Marcos River near Spring Lake in San Marcos, Texas to complete a lab activity.
In the field, my students gathered data on river width (W) using a retractable measuring tape. They divided themselves into teams and tackled five segments of the river on a 50m transect. Each group of 3-4 students chose a bank-to-bank section of the river for data collection.
Then, each group used a stadia rod to measure depth (D). They measured five equidistant points along a transect. Their first goal was to create a cross sectional depth profile of the river to see what the bottom of the river “looks” like. They plotted their depth measurements on a graph. The combination of five groups’ worth of graphing allowed us to see how the river floor changed, even over short distances.
Next, the groups each averaged their five bank-to-bank depth measurements to obtain a value for average depth (Da) and calculated cross sectional area (A) by multiplying W*Da.
From there, they used the floater method (in this case, a rubber duck) to calculate velocity (V). In this portion of the lab, the students all worked together to determine the amount of time it took for a rubber duck to float along a 50m river segment. Some students stood at the upstream point (a) to launch the floaters, others stood in the middle of the segment (on the bank) with cell phone timers, and other students stood downstream point to mark when the floaters crossed the 50m point (b). As a group, they floated the rubber duck down the river ten times and averaged their times together to obtain average velocity (Va).
Using the combined data from cross sectional area and velocity, they were able to calculate river discharge (Q).