An irrigator needs to know if sufficient water is available to open a ditch. A rafter would like to know if a certain river can be run. A reservoir manager needs to know when and how much water to release. A land planner for local government should know how much water is available for new growth. A wildlife biologist wants to know if a stream has enough water to support a fish population. All of these water users need a way to measure streamflow (also called discharge), the volume of water that passes through a stream cross-section.
Methods of measuring streamflow vary considerably, and depend on the size and speed of the stream and how often the measurement is needed. The method illustrated in the photo above gives a relatively accurate measurement of streamflow, but cannot be used to measure flow in a deep and fast-moving stream, and is impractical for providing continuous measurements at short time intervals (like minutes).
Water speed can be determined by simply floating an object along a stream and measuring the time it takes to travel a known distance. More accurate methods use propeller-type flow meters or very sophisticated doppler instruments that measure the velocity of fine suspended particles moving with the water.
Figure 1. A cross-section of a stream showing how streamflow can be measured. The flow or discharge through each subsection is determined by multiplying its area (depth x width) in square feet by its corresponding velocity in feet per second. The total streamflow in cubic feet per second (cfs) is then found by adding up the flows in all the subsections.
