Click here for the publication from the United States Geological Survey. Here’s an excerpt:
The most common method used by the USGS for mea- suring velocity is with a current meter. However, a variety of advanced equipment can also be used to sense stage and measure streamflow. In the simplest method, a current meter turns with the flow of the river or stream. The current meter is used to measure water velocity at predetermined points (sub- sections) along a marked line, suspended cableway, or bridge across a river or stream. The depth of the water is also measured at each point. These velocity and depth measurements are used to compute the total volume of water flowing past the line dur- ing a specific interval of time. Usually a river or stream will be measured at 25 to 30 regularly spaced locations across the river or stream.
Here’s the link to the USGS Water Watch website.
More USGS coverage here.