HISTORICAL CHANGES IN PRECIPITATION AND STREAMFLOW IN THE U.S. GREAT LAKES BASIN, 1915–2004

Precipitation and streamflow in the U.S. Great Lakes Basin increased during the last century. Total annual precipitation increased, on average, by 4.5 inches from 1915 to 2004 and by 4.2 inches from 1955 to 2004. Variability in precipitation from year to year was large, but there were numerous years with relatively low precipitation in the 1930's and 1960's and many years with relatively high precipitation after about 1970.

Mean annual runoff in the U.S. Great Lakes Basin increased, on average, by 2.6 inches from 1955 to 2004 in streams that were relatively free of human influences such as urbanization and storage regulation. Variability in runoff from year to year was large, but on average, runoff was relatively low from 1955 to about 1970 and relatively high from about 1970 to 1995. The mean annual 7-day low runoff (the lowest annual average of 7 consecutive days of runoff) increased from 1955 to 2004 by 0.048 cubic feet per second per square mile.

Runoff from 1955 to 2004 increased for all months except April. Precipitation and runoff during individual months from November through January, and also in July, increased by similar amounts. There were differences between precipitation and runoff changes during February, March, and April, which were likely due to lower ratios of snowfall to rain and earlier snowmelt runoff in recent years. Increases in precipitation were greater than increases in runoff for May, June, August, September, and October. Increases in evapotranspiration may explain all or part of this difference, however, some of the difference could be due to the different locations of the precipitation and streamflow stations in the basin.

Some of the few highly urbanized and highly regulated stations analyzed in this study had larger increases in annual 7-day low-runoff from 1955 to 2004 than any of the stations on streams that were relatively free of human influences. This demonstrates the human influence over time on very low streamflows in certain river basins.

Hydrologic changes—even over periods as long as 90 years—can be part of long-term cycles. Previous studies of Great Lakes Basin precipitation and St. Lawrence River streamflow, using data from the mid-1800s to the late-1900s, showed low precipitation and streamflow in the late 1800s and early 1900s relative to earlier and later periods.