Study Shows Rivers Warming in the U.S.

In a major new peer-reviewed study, a team of researchers collected all of the historical temperature data they could find on streams and rivers in the United States and discovered that water temperatures are increasing in many of our nation’s waterways.

According to the abstract for the paper, called Rising stream and river temperatures in the United States:

Water temperatures are increasing in many streams and rivers throughout the US. We analyzed historical records from 40 sites and found that 20 major streams and rivers have shown statistically significant, long-term warming. Annual mean water temperatures increased by 0.009–0.077°C yr−1, and rates of warming were most rapid in, but not confined to, urbanizing areas. Long-term increases in stream water temperatures were typically correlated with increases in air temperatures. If stream temperatures were to continue to increase at current rates, due to global warming and urbanization, this could have important effects on eutrophication, ecosystem processes such as biological productivity and stream metabolism, contaminant toxicity, and loss of aquatic biodiversity.

For those of you who aren’t scientists or water wonks, the abstract basically says that water temperatures in at least 20 major streams and rivers in the U.S. have risen from their historic norms. If we continue down our current path of urban development and greenhouse gas emissions, freshwater ecosystems will be disrupted and many of the fish, plants and wildlife that live in our rivers will be increasingly threatened. As the New York Times explains in a Green Inc. post:

Many streams and rivers in the United States are getting warmer, with the greatest increases in urbanized areas, according to research to be published in an upcoming edition of the journal Frontiers of the Ecology and the Environment.

Twenty major streams and rivers, including the Colorado, Potomac, Delaware and Hudson Rivers, are warming at statistically significant rates, the study found.

Increases in water temperature were often directly correlated to increases in air temperature and high levels of urbanization, said Sujay Kaushal, the paper’s lead author and a professor at University of Maryland’s Center for Environmental Science.

“We found the most rapid rates of increase in urban areas — this may be related to ‘urban heat island effects,’ from buildings, parking lots and pavements,” he said.

It should be noted that the study does not appear to present a conclusive reason for why warming is occurring, although it can obviously be inferred that urbanization (or more specifically, the urban heat island effect) and global warming are driving the warmer temperatures.

An important question is: how much is each one – urbanization or global warming – responsible for the observed warming? To figure this out will be complicated, and more studies will need to somehow compare stream and river temperatures in regions without significant land use changes. While the effects of urbanization on rivers are often observed immediately or over the course of a few years, the full impacts of our greenhouse gas emissions probably won’t be revealed for decades.

Luckily, the strategies to cool our rivers and streams are the same regardless of whether or not the climate or urbanization is causing the change. Again from the NYT:

“We need to pay attention to it and also think about mitigation and adaption strategies while we have this chance in terms of greening,” Dr. Kaushal said.

Those strategies might include planting trees to shade streams and cool urban areas. Increasing wastewater recycling and thereby reducing withdrawal of water from rivers and streams will also help keep temperatures down, the paper reports.

Of course, the most critical strategy, Mr. Kaushal said, is one that politicians and scientists are struggling to grapple with: reducing greenhouse gas emissions.

Despite the prolonged nature of climate change impacts, scientists have already established that greenhouse gas emissions are influencing the flow of rivers around the world. Although the threats of urbanization are certainly real and need to be addressed through low impact development (for more on the benefits of LID, see here or here) and other strategies, the most important way to protect our rivers, lakes and streams is to combat global warming by reducing greenhouse gas emissions.