The USGS Water Science School
In this picture, a hydrologist is making a stream-discharge measurement from a bridge. You can see by the length of the bridge that the river is too big to sample by standing in it, as is done when making a discharge measurement of a small stream. In this case, the technician divides the width of the river into about 25 equal cross-sectional points and takes a depth and velocity reading at each point. The crane is on wheels and is rolled along the bridge. The crane has weights at its base near the wheels to keep it from tipping over the bridge when the current meter device is lowered into the water.
The equipment he is using was very commonplace for decades. The crane is lightweight aluminum and folds so it can be placed in the truck. The reel contains a spool of wire cable that the measuring equipment hangs on. The device is lowered into the river by either a hand crank (going down) and/or and electric crank (coming up). At the end of the cable there is a weight (the thing that looks like a plane without the wings), with fins to keep it pointing upstream. Above the weight is the current meter, also with fins. The current meter has a set of cups that spin in the stream current. The faster the current, the faster the meter spins. Every time they spin a turn, a wire touches an electrical connection that transmits a clicking noise to the operator, who is listening with headphones. The number of clicks per minute can be translated into river-water velocity. Different river velocities are calculated for many portions (both vertically and horizontally in the river), and by doing some calculations, the amount of water flowing past the bridge each second can be computed. The amount is usually expressed in cubic feet of water per second.
One more thing -- notice that the scientist looks pretty calm here. First, it isn't the middle of the night, it isn't raining or snowing, and there's a nice, wide shoulder on the road to stand on. He would not be so calm taking a measurement from a two-lane bridge at night with tractor-trailer trucks barreling down in his direction!
As we said, the manual method of computing stream discharge has been around for decades—since before the advent of computers. But, with modern computers and electronics, it is now possible to hook up a computer to a device that constantly measures depth and velocity across the stream using sound waves. Acoustic Doppler current profilers (ADCP's) transmit sound into the water and receive reflected sound (echoes) from particles suspended in the water and from the streambed. Because an ADCP can measure water velocities, depth, and platform path simultaneously, it can compute discharge. Computing discharge using these methods is easier, faster, more accurate, and produces a much more detailed analysis of streamflow characteristics. The measuring device can be attached to a boat, pulled across the stream from shore, or be attached to a small, remote-controlled float that looks like a small speedboat (see picture).
Measuring streamflow Streamflow and stream gage heights