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Tree rings show evidence of droughts, floods along the Potomac River

350 years of data aligns with historical documents, study shows

Tree rings in the Potomac River watershed show evidence of severe droughts and floods over the last 350 years – and scientists can pinpoint the years of those events well enough that they align with writings from people that include Thomas Jefferson.

The findings, published earlier this month in the journal Water Resources Research, could help water managers predict water shortages and floods in Washington, D.C.

“We’ve got these Jefferson quotes and other qualitative records – we have somebody saying the drought was really bad,” said Jim Stagge, lead author of the paper and an assistant professor of civil, environmental and geodetic engineering at The Ohio State University. “But as an engineer, the next question is, how bad was it in terms of numbers? That’s what we wanted to find out. Because if we know how bad it was, we might be able to predict how bad it could be in the future.”

The Potomac River is the primary water source for Washington, D.C., and its suburbs in Maryland and Virginia, a metro area of about 4.6 million people that is predicted to grow by approximately 23% over the next 20 years, increasing demands on the water system. Drought and flood records go back only about 100 years, limiting information on extreme weather that could directly affect the amount of water available to the nation’s capital. This study, designed to predict water flows and shortages in the Potomac over time, suggests that tree rings could be one key to managing that water supply in the future.

“People talk about 100-year floods, but it’s a problematic concept, because if things are random, you could have three of those in four years,” said Max Torbenson, a postdoctoral researcher in Stagge’s laboratory and co-author of the study. “By using tree rings, we can build those records out to 300 or 400 years. The more data and the longer records we have, the better we can try to prepare for future extreme scenarios.”

The number of tree rings can tell how old a tree is, but the thickness of those rings also says something about the amount of water available in a given year. Wider rings mean the tree received more water that year. Thinner rings mean water was scarce.

Stagge previously worked with the Interstate Commission on the Potomac River Basin, an organization devoted to protecting the Potomac watershed and improving quality of life for people who rely on the river and its tributaries. Stagge and Torbenson are now working with the ICPRB to incorporate these findings into their long-term planning.

Tree rings are one way to extend flow records that water managers need: Data from those trees goes back around 350 years, more than three times the history offered by modern gauges.

Stagge and Torbenson gathered data about the Potomac River from the U.S. Geological Survey, which measures streamflow at points across the United States. For the Potomac River, they focused on measurements at Point of Rocks, Maryland, the most important gauge used to make water decisions for Washington, D.C. Then, they pulled tree-ring data from the International Tree-Ring Data Bank.

They evaluated records from more than 5,000 trees, and divided their data into annual seasons – winter, from December through February, and summer, from June through August.

The tree rings showed several years of consecutive drought – marked by very thin rings that indicated low water flows – that were similar to the 1960s drought and that had not previously been part of the conversation around water management. The data showed a drought in the 1960s that had been well-documented, but it also showed a drought that lasted from 1819 into 1820, about which Jefferson wrote that the “fatal effect has been greatly aggravated in this state by an unexampled drought, which having prevailed from June last to this time, destroyed the bread of that year, and threatens that of the present.”

The rings also showed very wide rings indicating high water flows in 1889. When the researchers consulted historical records, they saw that floods caused more than $1 million in damage to property on the C&O Canal between Cumberland, Maryland, and Washington, D.C. Those floods, too, had not previously been part of the strategies around water management.

Stagge said the study shows that tree rings can and should be used to help regions manage their water supplies – not to replace existing records, but to augment them.

“We can use trees to make sure that what we build today is much more robust and can stand up to some of these events,” he said. “They very well could reoccur; we just haven’t seen them in the span of a human lifetime.”

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