Low Flows in the Southeast

Research indicates low flow trends in Southeastern streams and rivers

Institute for Resilient Infrastructure Systems (IRIS) researchers Tim Stephens and Dr. Brian Bledsoe, Director of IRIS and Professor of Engineering, discovered a trend toward lower stream flows in the Southeastern United States that could impact everything from stream ecosystems to human reliance on healthy waterways. 

While streamflows are variable by nature, extremes caused by urbanization and industrial water use can be detrimental to ecosystem health. 

“Lower streamflows can result in elevated stream temperatures, lower dissolved oxygen levels, and ultimately a reduced amount of suitable habitat for aquatic organisms that are an integral part of the ecosystem.  An impact on one species can have a ripple effect throughout the ecosystem,” Stephens said. 

Additionally, low stream flows may elevate concentrations of harmful pollutants, which harm stream life and can pose threats to humans that rely on streams and rivers for drinking water and other uses. 

“Many processes, such as energy generation, require water supply infrastructure with an adequate amount of streamflow for operation. A decline in low flow levels could make it so that those processes are no longer operational,” Stephens said. 

An additional complicating factor is that streams and rivers around the country act in unison with waste treatment infrastructure to filter pollutants from industrial plants and communities. 

“Our river systems are an integral part of our waste treatment infrastructure: industries and communities discharge waste water into rivers, which naturally filter, or assimilate it. But low flows alter the proportion of waste to water in the river. If you’re putting in the same amount of a pollutant into a river that has a lower volume/flow in it, the altered concentration will have negative impacts on river health . We use these low flows to make decisions under the clean water act about how much pollution rivers can handle. The lower the flow, the less waste we can release, because there’s less capacity for the river to assimilate it,” Bledsoe explained.

Understanding how flow trends have changed over time is incredibly important for determining how much waste industries and communities can safely release. 

“A super important application of these kinds of analyses is looking forward into the future at the assimilative capacity of our rivers to clean and absorb waste. Further, understanding how trends might change will help guide future technologies, so that we can protect our communities and environments.”

While many factors contribute to the amount of water flowing through a stream or river at any given time, the researchers pinpointed several key variables. 

“There are many factors that can lead to lower flows in streams and rivers,” said Stephens, “For instance, changes in precipitation amounts or intensity can alter the amount and rate of water supplied to a stream, whereas increased impervious surfaces in cities can affect low flows by reducing the amount of water stored and transported below the ground’s surface.”

Additional factors include water consumption by industrial and agricultural processes, power production, residential use, increases in evaporation, and changes in vegetation such as the conversion of pasture to pine forest. 

To gather this data, researchers analyzed daily streamflow records at more than 300 locations across the southeast region during the time period of 1916 – 2016 to determine if low flows were increasing or decreasing with time. 

“Our analysis involved determining the magnitude and direction of trends (by how much and how fast low flows were going up or down), whether that trend occurred abruptly or was more gradual, determining the likelihood for the statistical test to falsely identify an insignificant increase or decrease, and a comparison among sites with and without significant human influence,” Stephens explained. 

The researchers found that the lowest stream flows declined in more than 80% of the analyzed records, and the rate of decline has accelerated in the last 25 years. Low flows also declined regardless of whether the location was considered unaffected by humans, affected by flow regulation, or impacted by other human influences, such as land use. In about half of the records, a decrease in annual low flows occurred abruptly around 1975 – 1985 or 1995 – 2005 as opposed to a more constant, gradual trend through time.

Declines in low flows were statistically significant at 30-60% of the locations, but the statistically insignificant trends may also be important. 

“We discovered there is usually more than a 40% chance that the statistical test incorrectly specified a trend as not significant, when it actually is—compared to a 10% chance that the test declared a trend as significant when it was not. Our study is the first to quantify the likelihood of declaring a trend is not significant when it actually is in low flow,” Stephens explained. 

Considering results that could be mistakenly declared insignificant is important; trends that are labelled insignificant are often ignored, which could cause decision makers and researchers to overlook valuable information with societal, economic and ecological implications. 

“The findings of our study underscore the need for adaptable and sustainable water management strategies, especially considering the societal, economic, and ecological impacts of competing uses for water resources during low flow periods,” Stephens said. 

This research was supported by the Southern Company and the NSF Sustainability Research Network Cooperative Agreement 1444758, Urban Water Innovation Network.