VOLUME 89 NUMBER 52 23 DECEMBER 2008 Dry Climate Disconnected the Laurentian Great Lakes Recent studies have produced a new of the moisture originates from humid air understanding of the hydrological history masses arising in the Gulf of Mexico and the of North America’s Great Lakes, showing subtropical North Atlantic Ocean. This inter- that water levels fell several meters below play of air masses maintains positive sur- lake basin outlets during an early postgla- face water balance, sustaining major rivers cial dry climate in the Holocene (younger and shipping canals between lakes, as well than 10,000 radiocarbon years, or about as discharge to the St. Lawrence River (Fig- 11,500 calibrated or calendar years before ure 1b). Although researchers have tradition- present (B.P.)). Water levels in the Huron ally assumed the Great Lakes to have been basin, for example, fell more than 20 meters hydrologically open since their formation below the basin overflow outlet between after 16,000 radiocarbon (19,200 calibrated) about 7900 and 7500 radiocarbon (about years B.P. during retreat of the last ice sheet 8770– 8290 calibrated) years B.P. Outlet riv- [Larson and Schaetzl, 2001], this paradigm ers, including the Niagara River, presently of continuous, abundant water supply is falling 99 meters from Lake Erie to Lake now known to be false. Mounting evidence, Ontario (and hence Niagara Falls), ran dry. described below, shows that hydrologically This newly recognized phase of low lake lev- closed lakes (i.e., lakes having water lev- els in a dry climate provides a case study for els below topographic outlets) existed in evaluating the sensitivity of the Great Lakes the Huron and Michigan basins, and likely to current and future climate change. in other basins, approximately 7900 radio- carbon or 8770 calibrated years B.P. The Laurentian Great Lakes New Understanding of Great Lakes History Collectively, these Great Lakes consti- tute one of the largest surface reservoirs Previous interpretations of past Great of freshwater on Earth. The lakes con- Lakes water levels lacked key information tain 23,000 cubic kilo meters of water, of on the altitude of outlets during glacial Fig. 2. (a) Lake level (blue) in Huron basin compared with the outlet elevation at North Bay which less than 1% is replenished annually rebound. Consequently, evidence for low- (gray) showing hydrologically closed lowstand when the lake fell by more than 20 meters below 14 14 by precipitation. Situated in the east cen- stands (lakes below present level) was taken the basin outlet after 8000 C (~8890 calibrated) years B.P. (b) Lowstand shorelines ~7900 C tral part of North America and shared by to indicate that lakes overflowed more north- (~8770 calibrated) years B.P. are shown in black, and present shorelines appear white in this colored watershed relief map. Lake basins: H, Huron; G, Georgian Bay; E, Erie; O, Ontario. (c) Tem- the United States and Canada (Figures 1a erly outlets that were still depressed isostati- perature and moisture trends relative to present conditions (dashed lines) for the southeastern and 1b), the five lake basins and their eco- cally by the previous ice load [e.g., Hough, Great Lakes region [Edwards et al., 1996]. Large black arrows indicate the time of the lowstands. systems support a population of more than 1962]. However, a greater understanding has 33 million people (in 1990– 1991) and host emerged from new empirical evidence and well- developed industries for shipping, fish- more precise analysis and modeling of iso- 1a) [Teller and Leverington, 2004]. Following residual meltwater and groundwater drain- ing, recreation, and power production, as static rebound rates in the Great Lakes Basin the retreat, the Great Lakes were then sup- age in the lowstands. well as water supply and wastewater dis- (GLB). The evidence includes newly dis- plied only by precipitation falling within the posal for municipalities. The overall range covered lowstand indicators in Lake Huron GLB, as at present. Explanation of the Lowstands of monthly water levels in all lakes during and Georgian Bay, such as submerged tree That the low- level lakes in the Huron and the past century has varied by less than stumps in situ and small lakes uplifted and Georgian Bay basins were closed hydrologi- Lakes without overflow, such as these 2.1 meters, and thus lake levels appear to isolated from the larger water bodies [Lewis cally is indicated by microfossil evidence lowstands in the GLB, can only be be relatively stable; yet even this amount of et al., 2005] and low- level erosion [Moore of brackish conditions (signifying excess explained by a dry climate in which water variation causes economic stress. et al., 1994]. evaporation) and a pause in outlet basin lost through evaporation exceeded water The interplay of three air masses (Fig- This body of evidence now confirms that sedimentation (suggesting cessation of river gained from direct precipitation and catch- ure 1c) currently controls climate and water lowstands occurred approximately from outflow). A lowstand also occurred in the ment runoff. The dry regional climate that levels in the Great Lakes watershed; most 7900 to 7500 radio carbon (8770–8300 cali- Erie basin at the same time, as indicated by forced the lakes into hydrologic closure brated) years B.P. in the Michigan, Huron, a submerged beach, while a shallow- water must have been substantially drier than and Georgian Bay basins (Figure 2a) [Lewis environment in Lake Ontario [Duthie et al., the present climate. Hydrologic model- BY C. F. MI C HAEL LEWIS , JOHN W. KING , STE F AN et al., 2005, 2007]. These hydrologically 1996] suggests a correlative lowstand in that ing of the present lakes shows that cur- M. BLAS C O , GREGORY R. BROO K S , JOHN P. closed lakes formed when water supply to basin as well. Acoustic profiling data show rent mean annual precipitation would have COA K LEY , THO M AS E. CROLEY II, DAVI D L DETT M AN , the GLB was greatly diminished by north- that submerged beaches of low- level lakes to decrease by about 25% in the Superior THO M AS W. D. ED WAR D S , CLI ff OR D W. HEIL JR., ward retreat of the Laurentide Ice Sheet. The also exist in the Superior basin [Wattrus, basin and by about 42% in the Ontario J. BRA df OR D HU B ENY , KATHLEEN R. LAIR D , retreat allowed discharge from Glacial Lake 2007]. A partial reconstruction of lowstands basin, in conjunction with a 5ºC mean tem- JOHN H. MCAN D REWS , FRAN C INE M. G. MCCARTHY , Agassiz to bypass the GLB into Glacial Lake is shown in Figure 2b; lowstand recon- perature increase, to achieve lake closure BAR B ARA E. ME D IOLI , THEO D ORE C. MOORE JR. Ojibway and drain via the Ottawa River val- struction is part of ongoing research. This [Croley and Lewis, 2006]. Paleoclimate sim- DAVI D K. REA , AN D ALISON J. SM ITH ley directly to the St. Lawrence River (Figure research also includes the assessment of ulations and reconstructions using pollen [Bartlein et al., 1998] and stable isotopes [Edwards et al., 1996] indicate that climate in the GLB (Figure 2c) was drier than pres- ent at 7900 radiocarbon (8770 calibrated) years B.P. Termination of the lowstand epi- sode about 7500 radiocarbon (8300 cali- brated) years B.P. coincides with the onset of a wetter climate as atmospheric circula- tion adjusted to the rapidly diminishing ice sheet [Dean et al., 2002] and moist air mass incursions from the Gulf of Mexico became more frequent. The discovery that the Great Lakes entered a low- level phase without having connecting rivers during the early Holo- cene dry period demonstrates the sensitivity of the lakes to climate change. The closed lakes of this phase occurred when the Great Lakes entered their present nonglacial state. Thus, the closed lakes afford a valuable example of past, high- amplitude, climate- driven hydrologic variation upon which an Fig. 1. (a) Location of the Great Lakes (blue) in eastern North America. Glacial Lake Agassiz (labeled A) is shown ~8000 14C (~8890 calibrated) improved assessment of lake- level sensitivity years before present (B.P.) when ice retreat caused its overflow drainage to switch from the Superior basin of the Great Lakes (red arrow) to Gla- in response to future climates can be based. cial Lake Ojibway (O) and the Ottawa River valley (black arrow). (b) The Laurentian Great Lakes and their watershed. (c) Major airstreams in North America with most water supplied to the Great Lakes by the moist air (tropical) from Gulf of Mexico [Bryson and Hare, 1974]. Laurentian Great Lakes cont. on page 542 541 EOS VOLUME 89 NUMBER 52 23 DECEMBER 2008 Hough, J. L. (1962), Lake Stanley, a low stage of Wattrus, N. J. (2007), Evidence of drowned paleo- Laurentian Great Lakes Lake Huron indicated by bottom sediments, shorelines in western Lake Superior, paper cont. from page 541 Geol. Soc. Am. Bull., 73, 613–620. presented at the 50th Annual Conference of the Larson, G., and R. Schaetzl (2001), Origin and International Association for Great Lakes Research, evolution of the Great Lakes, J. Great Lakes Res., Penn. State Univ., University Park, 28 May to 1 June. Acknowledgments parisons with paleoenvironmental data, Quat. 27, 518–546. Sci. Rev., 17, 549–585. Lewis, C. F. M., S. M. Blasco, and P. L. Gareau We are grateful to P. F. Karrow (University Bryson, R. A., and R. K. Hare (1974), The climate (2005), Glacial isostatic adjustment of the Lau- Author Information of North America, in of Waterloo), D. L. Forbes and S. St. George Climates of North America, rentian Great Lakes basin: Using the empirical edited by R.