Measuring the Streamflow of River Water for Whitewater Rafting
Questions that river guides hear all the time: How much water is going down the river on a rafting trip?
How is it measured?
River rafters often hear their river rafting guides throwing out numbers before or during a rafting trip… “the river is awesome today – it is at 6.4 feet” or “the river is at an average Summer flow of 1,000cfs”. What do these numbers mean? Where do the rafting guides find out this information?
Streamflow is measured in the U.S in cubic feet per second (cfs). One cubic foot of water is 7.48 gallons of water. A gallon of water weighs 8.35 pounds. So, one cubic foot of water weighs over 62 pounds. From the above example, a flow of 1,000cfs means over 62,000 pounds of water is flowing downstream every second!
Information on flows in cubic feet per second can be found online at the U.S Geological Survey or on a website dedicated to streamflow such as H2OLine. Sometimes flows are posted at the sites of dams and popular put-in sites for river recreation.
On a river where river rafting guides use a foot stage for measurement, things need an extra calculation. Stream stage measured in feet is the height of the water surface above an established altitude where the stage is set at zero. The zero level is somewhat arbitrary but often is set close to the streambed. Visual foot gages can be found on many rivers where recreation is popular. On many trips a guide will float near a visual gage such as the one seen below to check out the flow that day. Knowledgeable river guides can then make adjustments to their plans for running rapids based on this knowledge.
Behind the scenes on a river trip where guides are using a stream stage, there is also calculation in cubic feet per second. The precise amount of water flowing down a river is critical to the U.S Geological Survey and is still calculated in cubic feet per second. So, on a whitewater rafting trip where the guides are calling for a great day at 3.9 feet, the USGS sees a number such as 3,418cfs. These two numbers give each group the info they need – exact numbers for the scientists and a reference point for the local whitewater guide.