Rivers Heating Up With Warming

THE GIST:

• Around the country, rivers and streams are getting warmer.
• Rivers near urbanized areas are heating up the fastest.
• The trend threatens both animals and people.

Along with warmer air and warmer oceans, rivers also seem to be heating up with global warming. Across the United States, a new study found, water temperatures in some rivers have risen by more than 3 degrees Celsius in the last few decades.

Warmer river conditions could threaten both the biodiversity of waterways and the livelihood of people who drink out of them and live near them, especially in cities where heat-island effects accelerate warming.

"Even modest changes in temperature can have big biological effects," said lead author Sujay Kaushal, an ecologist at the University of Maryland Center for Environmental Science in Solomons.

"We're seeing the fastest rates of temperature increase in the most highly urbanized areas," he added. "That leads us to believe it's a one-two punch of global warming and development."

Kaushal and colleagues pieced together all the long-term water temperature data they could find for 40 streams and rivers around the country. Sources included old drinking water records, monitoring programs, and water-quality studies conducted by the United States Geological Survey. Information dated back, in some cases, to the turn of the 20th century.

In 20 of the rivers they looked at the data clearly showed warming, ranging from about 0.01 degree to 0.08 degree C per year, the researchers reported in the journal Frontiers in Ecology and the Environment. Those are small numbers, but the fractions added up over time -- to as much as a 3 degree C rise in Maryland's Patuxent River in less than 60 years, and probably more in other places.

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Fastest rates of warming occurred near urban areas in the Mid-Atlantic States, including along the Potomac River outside of Washington, D.C., the Delaware River near Chester, Penn, and the Patuxent River. Temperatures have also been rising in streams and rivers throughout the West, Midwest and Southeast.

Kaushal suspects that warmer air is partly responsible for warmer river temperatures. Development in cities probably contributes by removing shade trees from waterways. Paving green spaces with asphalt also leads to more heat reflection, which boosts the temperature of air and water even more.

For the food chain, consequences could be catastrophic. Researchers in the United Kingdom have predicted that insect numbers in some rivers will drop by 21 percent with every 1 degree C rise in water temperature. Other worries include species extinctions, invasions of alien species, and blooms of algae that could reduce the quality of drinking water and make recreation less enjoyable.

To give rivers a break, Kaushal said, city planners might want to think about planting more trees around urban rivers and making efforts to reduce the islands of heat that build up around cities. Most of all, he said, people need to reduce greenhouse gas emissions.

In fact, said limnologist Jon Cole of the Cary Institute of Ecosystem Studies in Millbrook, New York, the new study's biggest contribution might be the way it illustrates a clear warming trend in rivers in such a wide variety of places. Due to the unpredictable nature of weather, air temperature records are far more variable.

"Every once in a while, there are climate change deniers who try to pick apart various records," Cole said. "This is yet another line of evidence that the analyses are right. We know the world is warming. It's undeniable."