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.