Boaters frequently overestimate river currents, especially during times of high water. People sometimes mention mile-per-hour (mph) speeds with two digits, but actual measurements are far from that.

Part of the problem is that agencies that monitor rivers, like the U.S. Army Corps of Engineers (USACE), the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Geological Survey (USGS), don’t measure mph, which is a measurement of velocity. Rather, they measure volume, usually expressed in cubic feet per second (cfs). Further, there is no formula that converts cfs to mph.

But by comparing different rates and crunching some numbers, these and other agencies can derive an approximate value for mph that is within perhaps .5 mph of actual speed.

The Water Control Section at the Rock Island, Ill., USACE district offices calculated, for example, that at one point when the Mississippi was cresting this summer, it was running about 3.34 mph.

On April 19, when the Illinois River was peaking, the USGS measured the speed at the Marseilles Lock and Dam at 3.7 mph. Downstream at Henry, Ill., a measurement at the peak produced a reading of 3.4 mph.

A final, unofficial way to measure current velocity is to walk along a riverside path and judge your speed against the drift in the river. An average walking speed is about 3.1 mph.