Pick Our Brain - October 2010
How do you measure flow (in the Sacramento River)?
Up until the early 1990s, flow in the Sacramento River was estimated from a “rating curve” that related water stage at Freeport to flow. Each point on the rating curve required a cumbersome field estimate of flow in the cross-section using cables and analog current meters. Since that time, the U.S. Geological Survey (USGS) has employed acoustic Doppler current meters that emit acoustic pulses into the water column and measure the time it takes for the echoed signal to return.
Known as the “Doppler shift,” the frequency of the returned signal is affected by the current speed in the same way that a siren has a higher pitch as it approaches and a lower pitch after passing. The USGS flow meters use this principle to estimate water velocity simultaneously at many points in the cross-section. Flow is measured in English units as cubic feet per second (cfs). One cubic foot is equal to about 7.5 gallons—the inside space of some mid-size microwave ovens—and a flow of 1 cfs would fill a football field-sized pool to a depth of about two feet in one day. Since 1955, the average flow in the Sacramento River at Freeport is about 23,000 cfs.
Sacramento River flow at Freeport between 2002 and 2009 (Data from USGS Acoustic Doppler Current Meter)