Why Measure Rivers?
Why should we study rivers and streams? Rivers and streams are important water sources. People use river water for drinking, household uses, agriculture, energy production, and more. But communities must be careful when using water from rivers, because taking too much to quickly can impact other communities and ecosystems downstream. Hydrologists can collect stream measurements to help ensure that we are not using our water resources too quickly or in unsustainable ways.
Rivers are also facing big issues – including impacts from climate change. Rivers fed by spring snowmelt and seasonal monsoons are particularly susceptible to the impacts of climate change. Climate change affects the amount of snow that builds up in the winter and the timing of melt and seasonal monsoons. Pollution is another key issue for rivers. As runoff makes its way over the land, it crosses many surfaces such as roadways, lawns, and farm fields. Chemicals and waste from these surfaces are swept up by the running water and become concentrated in streams and rivers. Scientists and water managers work hard to understand and find solutions to the issues that rivers face today and promote healthy streams and rivers!
Measuring river width and depth
Water depth and width are common stream measurements. The depth and width may vary dramatically as you move upstream or downstream along any stream or river. Width expands as a river moves across flat plains and decreases as the river enters narrow valleys and canyons. Additionally, stream depth typically varies as you move from one side of a stream to the other. The deepest part of a stream cross section, also known as the thalweg, is not always in the middle – it is sometimes off to a side.
Depth and width measurements can be used to estimate cross-sectional area in a location of interest on a river. Cross-sectional area is the area of an imaginary flat surface perpendicular to the direction of flow, bounded by the streambed and water’s surface.
Calculating River Velocity
Velocity is the speed that the water is flowing. There are many great tools that hydrologists can use to estimate velocity with high precision and accuracy. However, you can estimate a stream’s velocity in a more low-tech way with just a stopwatch and a stick. Simply time a small item as it floats down a measured length of stream. Then divide the float distance by the time it took the stick to float that far to get velocity!
Velocity is important to know for river safety and boating. Velocity is also used to calculate discharge.
Measuring River Discharge
Discharge tells us about the amount of water flowing through a river. More specifically, discharge is defined as the volume of water that flows through some cross section of a river, per unit time. Think of it this way – velocity is a distance per unit time, and discharge is a volume per unit time. A small creek and a huge river that have the same velocity would not have the same discharge. Discharge is a measurement that allows us to see the difference between that small creek and huge river because it is a volumetric flow rate.
Measuring discharge helps us keep track of water resources for uses like drinking and growing food. Monitoring river discharge over many years allows water managers to predict and prepare for floods and droughts.
Field Trip Activity: Calculate stream discharge!
- Tape measure
- Meterstick or yardstick
- Small floating item (e.g., a stick)
- A net to catch the floating item (optional)
- A small, easily-wadable stream
- A buddy or two
Part 1: Estimate Velocity
You can estimate a stream’s velocity in a low-tech way with just a stopwatch and a floating object (e.g., a stick). Simply time a small item as it floats down a measured length of stream.
- In a small, easily wadable stream, use the tape measure a short float length (5-10 meters, or 15-30 feet) along the bank, parallel to the stream. Use a shorter distance if the stream is flowing very slowly.
- At the upstream end of your measured float length, drop a stick or other small floating object into the water. Drop the stick into a part of the stream that is flowing, not into a pool or eddy.
- As you drop the stick, start the stopwatch.
- Wait for the stick to float the measured float length. You may need to start over at step 3 if the stick becomes stuck.
- When the stick reaches the downstream end of your float length, stop the stopwatch. Record the time in seconds. (If the item you are floating is human-made like a paper boat or rubber duck, be sure to retrieve it so that it doesn’t become litter!)
- Repeat steps 2 through 5 two more times. Calculate the average time by adding the three times together and dividing that number by 3.
- Divide the float length by the average time to find the velocity.
Part 2: Find Discharge
Discharge is the flow of water in a stream. You can calculate discharge by multiplying the velocity of the stream by the approximate cross-sectional area of the stream.
- Return to the same place where you measured the velocity in part 1. In this same place, measure the width of your stream. Hold one end of the tape measure and toss the other end across the stream to a partner on the other side of the stream. Record the width from water’s edge to water’s edge.
- Use a meterstick or yardstick to measure the depth of the stream in three different places along the tape measure spanning the stream. For example, measure the depth near the left side, near the right side, and close to the center of the stream. Take the average of the three depth measurements.
- Multiply the average depth by the width to find the approximate cross-sectional area of the stream.
- Multiply the cross-sectional area by the velocity (from part 1) to find the discharge.
- Note: use the same units for all the numbers when doing calculations. E.g., if your velocity is in feet per second, then be sure to measure width and depth in feet.
Trying the activity with your class?
You’ll find a complete set of worksheets, activity guides, and visual aids for this activity and more cool hands-on science activities in my Science on the River Unit! (Plus you’ll support Wild Earth Lab with your curriculum purchase – which is pretty cool too😉).
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