The study, which analyzed 234,727 satellite images from 1984 to 2018, found that large river systems have seasonal color patterns related to their flow rates, and that the dominant color of one-third of U.S. rivers has changed significantly, although the direction of the color trend varied.
"What's causing those big changes to 33% of the nation's river systems is a question we weren't able to specifically answer in this study, but the sum of human activities — large agriculture, urbanization and increased organic matter — is changing river colors and water quality," said Mr. Gardner, an assistant professor of geology and environmental science at Pitt since 2020 and lead author of the report.
He said the data from Landsat satellite images of almost 34,000 river stretches covering more than 67,000 miles of rivers in the U.S. shows an association between color change "hot spots" and rivers flowing through urban areas or into reservoirs behind dams.
The peer-reviewed study, "The Color of Rivers," was published in December in the American Geophysical Union's online journal, Geophysical Research Letters. Most of the research was done during the past two years while Mr. Gardner was with the Department of Geological Science and the Global Hydrology Lab at the University of North Carolina-Chapel Hill.
The study report states that rivers are among the most imperiled ecosystems globally, but there is no "broad-scale understanding of their changing ecology because most are rarely sampled." Satellite imaging data like that used for the study could help fill in that gap and contribute to a better understanding of large river system health on a global scale, Mr. Gardner said.
According to the study, the human perception of a river's color, from brown to yellow to green to blue, has long been used as an indicator of a water source's suitability for consumption, recreation and aesthetic value.
The study classified 56% of the river stretches as yellow, 38% as green and only 6% of the rivers as blue. It did not analyze which color is preferred because that can vary with river history and use.
"We're not saying in this study whether a river is better because it's blue, yellow or green but rather, over time, if there's a trend or change in one direction or another," Mr. Gardner said. "We can make some generalizations, however. If a river is yellow or yellow-brown it contains more sediment. If it's green it contains more algae and less sediment. If it's blue, and there are very few, it's clear."
Rivers in southwestern Pennsylvania — the Allegheny, Monongahela, Conemaugh and Youghiogheny — display "a lot of yellowing patterns," Mr. Gardner said, including some hot spots, which are indicators of change from a variety of potential factors.
"The interactive map shows some red-shifted or yellow trends and some fast changes on river stretches in southwestern Pennsylvania," Mr. Gardner said. "There are some on the Allegheny, at the Allegheny Reservoir, upstream from Pittsburgh, and on the Monongahela and the Conemaugh."
Two areas experiencing significant color shifts during the past 35 years were the Tennessee River Valley, which moved from yellow to green due to more algae and less sediment, and the Columbia River basin in the Pacific northwest, which moved toward the blue-green end of the color spectrum.
"The change is slow, but widespread in those areas," Mr. Gardner said. "And both areas are heavily dammed."
He said a change in a river's dominant color from yellow to green is generally not a good sign because it signals more algae growth. A change from green to brown is "rare," he said, because most rivers in the U.S. and around the world are carrying less sediment today than they were three decades ago.
"The long-term record shows sediment in rivers is declining everywhere," said Mr. Gardner, whose research focus is on how rivers, lakes and landscapes move water, sediment and elements across continents. "If there's less or not enough, that can be a problem for coastal wetlands and deltas due to sea level rise."
While Landsat data has been used since the 1980s to assess the water quality and sediment loads in individual rivers or lakes over short time frames, the Gardner study is the first to use multiple Landsat missions to look at change across all large rivers in the continental U.S.
The first Landsat satellite was launched in 1972 to gather data on changes in land use, from forests to agriculture to urban areas, Mr. Gardner said. And in the 1980s, beginning with Landsat 5 and running through the current Landsat 8, sensors were added to assess water quality.
"Basically, any change in the color of a river a human might see, the satellite is able to pick up and say whether it's different than it used to be, We can use the satellite as an early warning system that has the benefit of a long-term data set," he said.
"For example, if there's a [coal] mine discharge enough to turn the Allegheny orange, the satellite can be applied to detect weird things that are going on and that we need to pay attention to."
While the research did not directly address climate change impacts, Mr. Gardner said, more rivers are expected to turn green from increased algal blooms and brown due to more organic matter seeping into surface waters.
"This study didn't specifically look at climate change impacts but rather at long-term seasonal patterns," he said. "But we have a related study in the works on lakes that focuses on that question, and we do see some seasonal shifts on lake color that could be climate related."
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