The total amount of water in the Great Lakes Basin is important in the long-term allocation of water to human use and to riparian and aquatic ecosystems. The water available during low-flow periods is particularly important because the short-term demands for the water can exceed the supply. Precipitation increased over the last 90 years in the U.S. Great Lakes Basin. Total annual precipitation increased by 4.5 inches from 1915 to 2004 (based on the average of 34 U.S. Historical Climatology Network stations), 3.5 inches from 1935 to 2004 (average of 34 stations), and 4.2 inches from 1955 to 2004 (average of 37 stations). Variability in precipitation from year to year was large, but there were numerous years with relatively low precipitation in the 1930s and 1960s and many years with relatively high precipitation after about 1970. Annual runoff increased over the last 50 years in the U.S. Great Lakes Basin. Mean annual runoff increased by 2.6 inches, based on the average of 43 U.S. Geological Survey streamflow-gaging stations from 1955 to 2004 on streams that were relatively free of human influences. 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. Runoff increased at all stations in the basin except in and near the Upper Peninsula of Michigan, where relatively small runoff decreases occurred. Changes in annual runoff for the 16 stations with data from 1935 to 2004 were similar to the changes from 1955 to 2004. 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 based on the average of 27 stations. Runoff in the U.S. Great Lakes Basin from 1955 to 2004 increased for all months except April. November through January and July precipitation and runoff increased by similar amounts. There were differences between precipitation and runoff changes for 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 larger than increases in runoff for May, June, August, September, and October. Some of this difference could be due to the different locations of the precipitation and streamflow stations in the basin. Part of the difference may be explained by changes in evapotranspiration. Some of the few highly urbanized and highly regulated stations analyzed in this report had larger increases in annual 7-day low-runoff from 1955 to 2004 than any of the stations in the U.S. Great Lakes Basin that are on streams relatively free of human influences. This demonstrates the human influence over time on very low streamflows. Changes-even over periods as long as 90 years-can be part of longer 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.