Aquatic plant growth affects stream gaging

It's that time of year when growth of aquatic vegetation (called "macrophytes") begins to fill the stream channel and raise the water depth, even without changes in river flow. Remember that streamflow is the volume of water that flows past a given point on the river per time unit. We usually measure streamflow in cubic feet (volume) per second (time). The depth of the river (measured in feet) is a function of both streamflow and the shape of the stream channel. As macrophytes fill in the channel with plant biomass, there is less room for the water in the channel. The only way for the same flow to pass through an effectively smaller channel is for the water depth to increase.  That is what is occurring right now in the river from Island Park Dam to Pinehaven.  Water depth is increasing independent of flow.

The result of this at the stream gaging station at Island Park Dam is that the relationship between river depth (called "stage") and flow (also called "discharge") is constantly changing. In practice, the gage station records stage, and that is automatically converted to discharge using the mathematical stage-discharge relationship. Because the U.S. Geological Survey updates the stage-discharge relationship only once every few weeks, the calculated discharge will differ from the actual discharge in between stage-discharge updates. In particular, discharge will appear to increase when macrophytes are growing, until the new stage-discharge relationship is calculated. This happened between July 4 and July 9, when depth increased about one-tenth of a foot due only to macrophyte growth.  This made it appear that flow was increasing--from about 1700 cfs to about 1800 cfs.  However, there was no flow change at the dam--the discharge was constant throughout this 6-day period.  In fact, flow out of the dam was reduced on July 9 by about 100 cfs, appearing to have reduced outflow from 1800 to 1700 cfs.  In reality, this change reduced flow from 1700 cfs to 1600 cfs.  On July 10, USGS personnel adjusted thje stage-discharge relationship and confirmed that the true discharge is really around 1600 cfs, not 1700 cfs.

To learn more details about stream gaging, stage-discharge curves, and the effects of macrophyte growth and senescence on the stream gaging station at Island Park, see the technical document I prepared on this subject in 2013 here.