488 J. Great Lakes Res. 29(3):488–500 Internat. Assoc. Great Lakes Res., 2003 Lake Level Response to Seasonal Climatic Variability in the Lake Michigan-Huron System from 1920 to 1995 Erin P. Argyilan * and Steven L. Forman Department of Earth & Environmental Sciences Institute of Environmental Science and Policy University of Illinois at Chicago 845 W. Taylor St. (M/C 186) Chicago, Illinois 60607-7059 ABSTRACT. Recent research indicates a shift in the timing and range of the seasonal lake-level cycle of Lake Michigan-Huron since 1860. The largest changes occur during the winter-spring transition. The objectives of this study are to (1) quantitatively assess seasonal variations in precipitation, runoff, and evaporation, (2) evaluate long-term trends in seasonal water supply to Lake Michigan-Huron, and (3) understand how variations in net basin supply at a seasonal timescale contributed to development of extreme lake levels recorded in Lake Michigan-Huron between 1920 and 1995. Total water input, which is the sum of overlake precipitation and runoff, has increased during autumn and winter seasons at rates of 14.6 and 9.3 mm/decade, respectively. A rise in autumn total input reflects an increase in overlake pre- cipitation (6.3 mm/decade) and runoff (8.3 mm/decade). The increase in winter input was driven by runoff (7.5 mm/decade) at the expense of decreasing spring runoff (7.8 mm/decade). This shift to an increasing dominance of winter runoff at the apparent expense of spring runoff is most pronounced post 1965 and suggests a hydrologic response to a warming climate. Extreme lake levels principally reflect either anom- alies occurring in one or more seasons or a directional shift over many years, but often with seasonal bias. Seasonal changes in overlake precipitation, runoff, and overlake evaporation lead to the formation and persistence of extreme lake levels. The effects of seasonal water supply anomalies depend on the coincidence to long-term decadal-scale variation in lake level. INDEX WORDS: Great Lakes, lake level, net basin supply, 20 th Century climate change, runoff, seasonal. INTRODUCTION The Laurentian Great Lakes are the world’s single largest reservoir of usable fresh water. As our knowl- edge and concerns regarding regional and global cli- mate change continually increase, it is critical to bet- ter understand how changes in water supply to the Great Lakes caused by climatic variability impact lake-level fluctuations. Since 1860 instrumental measurements have shown that water level in the Great Lakes has fluctuated within a range of approx- imately two meters (Fig. 1). The Lake Michigan- Huron system exhibits the greatest range of fluctua- tion of all the Great Lakes (Fig. 1b). Lake Michigan-Huron has been permanently lowered by 0.27 m since 1900 by three artificial channel changes * Corresponding author. E-mail: eargyi1@uic.edu in the St Clair River including dredging for commer- cial gravel removal between 1908 and 1924, and uncompensated navigation improvements for the 7.6 m and 8.2 m projects in 1933 and 1962, respectively (Derecki 1985). Nevertheless, Lake Michigan-Huron remains the least affected by engineering modifica- tions that influence lake-level, including inflows from upstream lakes and outflows to downstream lakes (Changnon 1987). Hence, we consider the Lake Michigan-Huron system to be sensitive proxy for understanding the relation between climate and water-level fluctuation within the Great Lakes. Water fluctuations in the Great Lakes reflect changes in the hydrologic water balance of the indi- vidual lakes expressed as: ± ΔS = P + R – E + Q I – Q o ±D (1)