Global evidence

Irrespective of its cause, the effects of global warming seem to be becoming facts of life. Scientists at the US Geological Survey (USGS) have found evidence in eastern North America that the snow is melting and running off into rivers earlier than it did in the first half of the twentieth century. They have found that winter-spring flows in many rivers in the northern United States and Canada are occurring between five and ten days earlier.

Glenn Hodgkins and Robert Dudley of the USGS Maine Water Science Center and colleagues studied rural, unregulated rivers with more than fifty years of USGS and Environment Canada river flow data. Some 179 rivers in eastern North America met the criteria of our study with 147 in the United States from the Dakotas to New England and 32 in Canada from Manitoba to Newfoundland, he explains. These rivers are sensitive to changes in precipitation and temperature, Dudley adds.

High spring streamflow on the St. John River in northern Maine (Source: USGS)

The team compared the dates by which half of the total volume of winter-spring runoff has flowed past a river gauging station during each year. They found that most rivers north of 44° latitude (southern Minnesota and Michigan through to northern New York and southern Maine) showed earlier winter-spring stream flows. Many of the stations south of this line in Iowa, southern Wisconsin, and northern Illinois had later stream flows, by contrast.

The researchers explain that changes in average monthly flows support their findings. A high percentage of rivers north of 44° latitude have increases in January, February, and March stream flows and relatively high percentages of rivers have decreases in May and June. In 2005, researchers from Scripps Institution of Oceanography and the USGS found earlier streamflow across large portions of western North America in rivers with significant snowmelt runoff.

Salmon

The documented changes in the timing of winter-spring stream flows in eastern North America could have important implications for life in aquatic ecosystems. However, the potential impact is not yet well understood. One possible impact may be that the survival rate of Atlantic salmon could change for the worse.

Cartoon by Peter Welleman, (http://www.cartooncreator.nl)

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