For generations, scientists have judged the quality of river water based on its color — whether it's blue, green, or brown can help to determine levels of pollution, algae, etc.
However, the scientific community has yet to widely use river color as a means to gauge the health of entire river networks over long time periods.
In a new paper, a group of researchers combined the value of intuitive naked-eye observation with data from decades of NASA satellite observations.
Analyzing river colors over decades
In their paper, published in Geophysical Research Letters, the team of scientists explains how they believe they are taking advantage of what they perceive as a missed opportunity over the years.
"Water color, as perceived by the human eye, is … an intuitive and broadly applicable water measurement," the team of researchers led by environmental scientist John Gardner from the University of North Carolina at Chapel Hill explains in their paper, via ScienceAlert.
"It is intuitive since it is based on human perception of color, and people often discern water's suitability for consumption, recreation, and aesthetic value based on water color."
The team combined this traditional method of river health appraisal with a much more wide-ranging outlook — mass data collected by satellites over decades.
By analyzing a massive dataset of almost 235,000 images from NASA and the USGS's Landsat program spanning 34 years up to 2018, the researchers measured and monitored the changing colors of all large rivers in the continental US.
The potential of mass river data observation
The results — which can also be viewed via an interactive website — show that the dominant color in one-third of American rivers significantly changed during the study period.
"Over the full record, 33 percent of rivers had significant trends in color, but the direction of the trend varied regionally," the team writes.
Overall, in 56 percent of observations, the dominant color was yellow, followed by 38 percent green. The trends depended largely on where the rivers were located.
"The prevalence of blue-shifted trends in the western USA and red-shifted trends in the eastern USA suggest there are regional drivers of long-term trends, including changes in hydroclimate (e.g. river flow), land use, and watershed management," the researchers write.
The researchers say that, though satellite imaging of rivers is a common practice, the scale at which they have done it can help to improve "water quality monitoring, identifying global hotspots of change, and advancing macrosystems ecology in rivers."
Such a tool could help to analyze the human impact on rivers over the coming years at a scale never before seen.