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Snow Is Melting Earlier in N. America, Study Finds

Richard A. Lovett
for National Geographic News
March 28, 2006
 
Is spring coming earlier these days?

Maybe—at least in places where "spring" can be defined as the time when snowmelt floods the rivers, according to a new U.S. Geological Survey study published March 21 in the journal Geophysical Research Letters.

Hydrologist Glenn Hodgkins and a colleague at USGS's Maine Water Science Center examined stream-flow records from 179 rural rivers in the northeastern U.S. and southeastern Canada.

The records are maintained via gauging stations, which measure stream flow by tracking water depth.

Only rivers that had been monitored for at least 50 years were used, and only relatively small ones (the average stream flows of larger rivers are more likely to have been artificially affected by the construction of dams). Some streams had been monitored for as long as 90 years.

In a prior study of New England rivers, Hodgkins had found that total annual flow had not significantly changed. In his recent study, he found that in many of the rivers north of 44° north latitude (where winter snow plays the biggest role) the time of peak runoff had shifted earlier by about five to ten days over the last 50 years.

Hodgkins's finding comes hard on the heels of a 2005 study that found similar results in western North America, where mountain snow feeds many rivers.

"We investigated 300 snowmelt-dominated rivers from California to Alaska," said Iris Stewart-Frey, who led the study for the Scripps Institution of Oceanography in La Jolla, California.

"We found that snowmelt is coming earlier over the past few decades."

(Related news: "Global Warming Is Rapidly Raising Sea Levels, Studies Warn.")

What Difference Does It Make?

The implications are a bit different in the East and the West.

In the West early snowmelt lengthens the dry season and puts extra strain on reservoirs that store water for the summer needs of a growing population.

And governments can't simply fill reservoirs to the brim.

That's because there is still the risk of late-spring rainstorms, and full reservoirs can't be used for flood control. This is of particular concern in arid regions, Stewart-Frey added by email.

"As the southwestern U.S. is already pushing its limits in the water-resource department, these shifts in stream-flow timing can have great implications for water supplies," she wrote.

In the wetter East this isn't as big a concern, but timing shifts may affect fish, such as Atlantic salmon, Hodgkins said. For example, an earlier snowmelt may signal juveniles to swim downriver and out to sea too early, when ocean conditions may not be right for them.

Global Warming

The big question, of course, is whether these changes are due to global warming.

"That's the question everyone asks," Hodgkins said.

It's not his field, he says. But he did note that the shifts he saw were more strongly correlated to air temperature than to other variables, such as the amount of snow or rain in a given year.

In the West, Scripps's Stewart-Frey can see at least three possible causes of earlier snowmelt in her data.

Two are from cyclical climate changes, such as El Niño and a longer-term pattern called the Pacific Decadal Oscillation. Both phenomena dominate climate on the West Coast.

The third cause appears to be global warming.

"We are already seeing the effects of climate change," she said.

Effects are easier to see in the West, she added by email, because precipitation is much more highly seasonal than it is in the East. This makes it easier to detect links between stream-flow levels and climate influences.

(Quiz: Grade Your Climate IQ.)

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