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.
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.
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