Western streams are running lower in late summer

The trend toward weaker-flowing rivers and streams in the American West in late summer has been going on since the late 1940s, according to a study by Iris Stewart, of Scripps Institution of Oceanography, and several colleagues.[[ Stewart, I. T., D. R. Cayan, and M. D. Dettinger. “Changes in snowmelt runoff timing in western North America under a ‘business as usual’ climate change scenario.” (PDF) Climatic Change 62, no. 1 (2004): 217-232.]] The evidence comes from streamflow gauges monitored by the US Geological Survey.  Dr. Steve Running, of the University of Montana, has documented a similar pattern within the state of Montana.

Since many Western rivers get a majority of their water from melting mountain snow, it is reasonable to guess that this drop-off has to do with changes in mountain snowpack — and in fact, that snowpack in the west is melting several weeks earlier than it did a few decades ago. This is likely related to the fact that the climate of the American West is getting warmer.

Among the consequences of reduced flow are drier forests, subject to an increase in harmful insect epidemics and in wildfires, and a reduction in habitat for trout and other fish.