USGS Newsroom

Editors: The report, Historical Changes in Precipitation and Streamflow in the U.S. Great Lakes Basin, 1915-2004 , is available online. This report and others on the Great Lakes are also available on our new website: National Water Availability and Use Program - Great Lakes Basin Pilot .

More precipitation has been falling recently in the Great Lakes Basin than in the more distant past.  During the past 90 years, total annual precipitation increased by 4.5 inches and much of that increase occurred during the most recent third of that time span. These are among the findings in a report from the U.S. Geological Survey (USGS) that describes streamflow and precipitation changes in the U.S. Great Lakes Basin.

"We saw, as probably anyone living in the Great Lakes Basin has, substantial variability in precipitation from year to year and season to season," said USGS scientist Glenn Hodgkins, who led the study. "But clearly, the basin has been receiving more precipitation than it did in the early 1900s."

Despite the increase in precipitation, the scientists determined that streamflow in the basin increased more modestly. During the past 50 years, an average annual precipitation increase of 4.2 inches resulted in an average runoff increase of 2.6 inches as measured at 43 USGS streamflow-gaging stations. The difference may be partly caused by increased evapotranspiration - water lost to the atmosphere through evaporation from water bodies and soil and transpiration from plants.

"This study is a key component in assessing water availability and use and in building a water census for the nation," said Robert Hirsch, USGS Associate Director for Water. "We now have a much clearer picture of the volume of water being added to the largest fresh-water system in the U.S. and how those volumes have been changing over the last few decades."

The Great Lakes Basin, which encompasses Lakes Superior, Michi­gan, Huron, Erie, and Ontario, contains 95 percent of the fresh surface water in North America and 18 percent of the fresh surface water in the world. Ground water underlying the basin constitutes another large volume of freshwater. It has been estimated that streams contribute 46 percent of the water that goes into the Great Lakes. Direct precipitation into the lakes makes up about 53 percent. The remaining 1 percent of water comes to the Great Lakes by diverting water from outside of the basin.

Among the findings in this report:

• Precipitation over the land part of the U.S. Great Lakes Basin was lower than at present for several decades-the average precipitation for 1915-1934 was 10 percent lower than for 1984-2003.
• Precipitation did not consistently increase for all months over the last 90, 70, or 50 years.  February and March precipitation declined slightly for all three periods. The largest increases occurred from July through October.
• In February and March, precipitation declined but streamflow increased; in contrast, April precipitation increased, but streamflow decreased, illustrating an earlier snowmelt runoff in recent years.
• Annual low streamflows increased more during the last 50 years in some of the few regulated and urban basins analyzed than in all of the relatively natural basins.

This study is part of the USGS Water Availability and Use Initiative, which began in 2005 at the request of the Congress with a pilot study of the Great Lakes. The focus of the Great Lakes Basin study is on improving fundamental knowledge of the water balance of the basin, including the flow, storage, and withdrawal of water by humans. A report released earlier this year described historic lake level changes and their ecological impacts. Reports on ground-water flow and storage, and water use in the region have also been published and several other reports are planned. These reports are intended to inform citizens, communities, and natural-resource managers of how much water we have, how water availability has changed in recent decades, and how much we will have for the future.

The Initiative will add other major water-resources regions in the future as funding permits. At full implementation, the program would include national synthesis component to provide an overview of the status and trends of the nation's water resources in forms useful to policy makers, public officials, and the general public.