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Streamflow Trends in the United States
Lins, H.F. and Slack, J.R.  1999.  Streamflow trends in the United States.  Geophysical Research Letters 26: 227-230.

The authors begin by noting that "floods and droughts cause more damage annually in the United States than any other natural disaster," and that "there is an increasing trend in both flood damage and drought vulnerability."  However, they also note that "most of the flood damage increase stems from continuing urban and suburban development on floodplains," and that "drought vulnerability increase is from development in regions of lower renewable water supplies."

It is perhaps understandable, then, that there is a widespread perception that, as the authors put it, "extreme hydrologic events are increasing in frequency and/or magnitude."  In particular, they cite the Mississippi floods of 1993, the widespread flooding of 1997 in the West, Upper Midwest and Ohio Valley, as well as the droughts of 1988 and 1995-96, as giving "rise to speculation that floods and droughts are increasing, possibly in response to greenhouse warming."  Another reason for this misperception could well be the highly publicized claims of the U.S. President and Vice President that these things are indeed happening and that they are in fact caused by global warming.

What was done
In an attempt to shed more light on this topic by looking at what real-world data have to say about it (see our "Show Us the Science" editorial), the authors analyzed secular trends in streamflow for 395 climate-sensitive streamgaging stations (including data from more than 1500 individual streamgages) in the conterminous United States, some of which data sets stretched all the way back to 1914

What was learned
The authors found many more uptrends than downtrends in streamflow nationally, with slight decreases "only in parts of the Pacific Northwest and the Southeast."  These results, they say, "indicate that the conterminous U.S. is getting wetter, but less extreme."

What it means
It is difficult (if not impossible!) to conceive of a better result.  As the climate has warmed over the past century, the United States has gotten wetter, in the mean, and less variable at the extremes, where floods and droughts occur.  Indeed, it makes us think of the words of a popular song that floats across the airwaves now and then: "I like it, I love it, I want some more of it!"

Reviewed 15 February 1999