Why are America’s rivers turning yellow? Shocking new study sheds light on the slow death of country's waterways
In one-third of US rivers, the color has changed significantly over the last three decades, according to a new analysis. Satellite observations over 35 years show that these rivers, which are over 60 meters wide, are dominantly yellow and green in color.
What is the study?
According to scientists, rivers can appear in different colors, and the water color is linked to water quality and can be related to the amount of sediment, algae, pollution, and dissolved organic carbon in water. “Humans can, therefore, discern waters’ suitability for use with our eyes. While we know many rivers are impaired globally, often due to poor water quality, the color of rivers has not been widely measured to investigate changes through space and time. Satellites act as “eyes in the sky” and regularly observe earth’s large rivers,” write researchers from the University of North Carolina at Chapel Hill, University of Pittsburgh, and Colorado State University.
Accordingly, using satellite remote sensing records from 1984-2018, the team measured the color of rivers across the US. In total, they collected an estimated 15.9 million measurements over the 34-year period on the 67,000 miles (108,000 kilometers) of rivers in the US. This allowed them to track significant trends in color change over time.
What did they observe?
More than half of rivers (56%) observed had changed to yellow, while 38% turned green. Only 6% of the rivers were found to be blue, reveals the study published in Geophysical Research Letters. “We found that the dominant color in one-third of rivers has significantly changed. Observations of water color can pinpoint rivers undergoing rapid environmental change and work towards continental-scale understanding of rivers,” they explain.
Rivers draining the Rocky Mountains and the lower Mississippi River were “red-shifted,” that is yellower. Rivers across the southeastern US were “blue-shifted on average,” implying that they were greener. Rivers in the northwestern US and the northeastern US tended to become greener.
“Most rivers did not have significant long-term trends in color. Rivers with variable color over time, but no trend, were common across the US, totaling 55% of rivers (49,734 km). 12% of rivers (11,191 km) had no trend and steady color over time. Steady patterns were found largely in the central US and Lower Mississippi River,” say authors. They add, “We identified 534 blue-shifted hotspots located largely in the northwestern US. However, the fastest blue-shifted rates occurred in the southern Great Plains. We identified 307 red-shifted hotspots located largely in the northeastern US (69 or 22%) and Ohio River Basin (62 or 20%). However, the fastest red-shifted rates occurred in the lower Colorado River Basin.”
River color also had distinct seasonal patterns. Summer red-shift was more common in the western US, likely driven by snowmelt driven flow and sediment pulses. Spring red-shift was more common in the eastern US.
Why are they changing color?
While the team is unsure about the exact reason behind the changes, they note it can be caused by algae or the sediment amount. According to them, connected lakes and reservoirs can be sediment traps and algal incubators, which is consistent with their observations that lakes were blue-shifted (greener) and rivers were red-shifted (yellower).
“Rivers of the southern Great Plains show the effect of both landscapes and lakes. Flowing from West (semi-arid) to East (humid) across aridity, soil, and vegetation gradients, the red-shifted color signature of upstream landscapes was erased as sediments settled in river corridors, perhaps within successive dams, and blue-shifted colors emerged,” the findings state.
The experts suggest that while regional climatic and landscape properties likely control the range of colors a river can be, in-channel effects such as dams and rapid landscape transitions can reset the color of rivers. “Fragmentation and transitions in color along rivers are important because the color is likely linked to ecological function. The observed spatial patterns in color may be indicative of primary productivity regimes, suspended sediment, as well as light availability for photodegradation,” they add.
According to the authors, the prevalence of greener trends in the western US and yellower trends in the eastern US indicate there are regional drivers of long-term trends, including changes in hydroclimate (for example river flow), land use, and watershed management. However, they hypothesize that local drivers are also important, especially in rivers that are hotspots of change. “Since large rivers integrate millions of kilometers of land area, understanding rivers and their impairments is inherently macroscale: both distant and local impacts generate the patterns we observe,” say experts.
Significantly, the satellite images reveal hotspots where human influences, such as dams, reservoirs, agriculture, and urban development maybe changing the color of some rivers. Among rivers within the top 25 percentile of rates (for example, faster rates of change), 45% are near a dam (that is, within 25 km upstream or downstream) and 22% are located within urban areas. However, among rivers within the bottom 25 percentile of rates (slowest rates of change), only 12% are near a dam and 7% within urban areas.
“To highlight examples of rapid change, water levels in Lake Mead on the Colorado River have declined since the early 2000s, which we observed as a shift in watercolor due to shrinking reservoir extent. The upstream end of the reservoir dried down transitioning from blue/green open water into a more yellow river. The reverse transition occurred on the Double Mountain Fork Brazos River in Texas. A dam was built in 1993 transforming a yellow river into a blue/green reservoir,” notes the team.
The experts emphasize that similar trend analysis can be used to identify hotspots of change, target rivers for future experiments, and disentangle drivers of change. “The intuitive and easily observable nature of watercolor could enable the collection of massive volumes of data across spatial and temporal scales for water quality monitoring, identifying global hotspots of change, and advancing macrosystems ecology in rivers,” they conclude.