US streams “less extreme hydrologically” than in 1940s

Floods have not become more severe on US streams despite increased levels of atmospheric carbon dioxide, according to research released on January 15 1999 by the US Geological Survey.

USGS hydrologist Harry Lins and USGS mathematician Jim Slack calculated trends for 395 climate-sensitive streamflow gauging stations in the US during the twentieth century, but found no evidence to support the view that an increase in atmospheric carbon dioxide is causing floods and droughts in the US to become more severe.

Lins and Slack note that “there has been considerable speculation as to whether or not an increase in atmospheric carbon dioxide – popularly referred to as greenhouse, or global, warming – is causing floods and droughts to become more severe. Based on our analysis of changes in long-term streamflow across the nation, we see no evidence that it is.”

Writing in the January 15, 1999 issue of Geophysical Research Letters, the USGS researchers said the general pattern has been toward increasing streamflow in most regions of the conterminous US since the 1940’s, and probably throughout the twentieth century. Decreases appear only in parts of the Pacific Northwest and the Southeast.

By analyzing changes in flow, from the lowest daily flows each year to the highest flows, Lins and Slack were able to get a more complete picture of streamflow trends over time.

“When we looked at the low to middle range of flows,” Lins said, “we saw a distinct upward trend. For example, if we take the lowest one-day flow, which is called the annual minimum flow, nearly one-third of the streamgauges that we analysed showed an increase.”

This same pattern was also true for the annual average, or median, streamflow. At the highest flows, however, only four percent of the gages experienced increases, while five percent showed decreases.

“We can draw three general conclusions from these trends,” Lins said. “First, the nation’s streams are carrying more water on average. Secondly, the streams are experiencing less severe hydrologic droughts, and thirdly, the streams are not experiencing more floods.

“In other words,” he said, “the United States is now less extreme hydrologically than it was earlier in the century.”

The USGS, in cooperation with more than 800 state, local and other federal agencies, operates a streamflow gauging station network, consisting of about 7000 stations across the country, to collect information about US water resources.

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