WASHINGTON — Global warming could increase flooding, shrink salmon habitat and invite in more invasive species in the West, scientists conclude in a sobering new report.
Snow will melt sooner, the report predicts. Rain will replace snow altogether in some places. Fisheries will stress out. Surface water will be harder to come by, and groundwater will be drained, as average temperatures rise.
"These changes pose a significant challenge and risk to adequate water supplies," Mike Connor, the head of the U.S. Bureau of Reclamation, declared Monday.
The changes are identified in a congressionally ordered study of global warming and Western water resources that Interior Department officials call the first of its kind. The 226-page study, issued Monday, examines eight Western water basins, including those served by the Colorado, Missouri and Columbia rivers.
Many other studies have traced the relationship between climate change and water supplies. This latest study, authorized in 2009, is unique for examining all the Western water basins that the Bureau of Reclamation serves. The bureau operates dams, canals and power plants in 17 Western states.
"There's already a lot of data out there," Connor said in an interview. "This reaffirms a lot of what we already know."
The new study anticipates average temperatures rising 5 to 6 degrees Fahrenheit through the 21st century. The federal scientists do not, however, wade into the political thicket of assessing blame for why the Earth is warming.
During the 20th century, scientists recorded a 3-degree increase in average temperatures through California's Central Valley and a 2-degree increase in Washington state's Columbia River Basin.
In general, higher temperatures will result in what scientists describe as "more frequent rainfall events (and) less frequent snowfall events." Even within regions, though, the changes will vary.
On the western rim of the Columbia River Basin, for instance, winter temperatures in the Cascades already hover close to freezing. With warmer temperatures, these snowpacks will shrink dramatically.
While avoiding firm predictions about future flooding, scientists say their studies "raise questions ... (about) potentially greater flood risk during the 21st century." These dangers include "more winter runoff" and "more extreme runoff events," the report says.
The precipitation changes probably will lead to "reduced water supplies" in Western reservoirs that rely on snowmelt during the late spring and summer, scientists warn.
In California's Sacramento-San Joaquin Delta, higher water temperatures are expected to lead to more delta smelt dying, while invasive species such as the quagga mussel will become more plentiful. The unwanted quagga mussel also is expected to proliferate in the Pacific Northwest, scientists say.
Salmon populations probably will suffer, as streams and estuaries warm up and the seasonal water flows change. In the Pacific Northwest, scientists likewise predict more "thermal stress" for salmon as freshwater temperatures rise.
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The Bureau of Reclamation report on water in the West
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