C. F. Michael Lewis

Great Lakes paleohydrology: Complex interplay of glacial meltwater, lake levels, and sill depths

The oxygen isotope record of ostracode and clam shells recovered from Great Lakes cores of known age allows definition of times when meltwaters from the Laurentide ice sheet were important components of lake water in the several lake basins since 12 ka. We find that the lowstands in Lake Huron and Georgian Bay are characterized by isotopically light waters (δ 18 O values of -20‰ to -22‰ relative to SMOW [standard mean ocean water]) and the highstands by isotopically heavy waters of more local origin. These data can be used to determine the degree of hydraulic separation among the early Holocene lakes. Southern Lake Michigan, for instance, may mix with northern-source waters only during times of rising and high water levels. Generally it is characterized by waters of local origin.

Dry Climate Disconnected the Laurentian Great Lakes

Recent studies have produced a new understanding of the hydrological history of North Americas Great Lakes, showing that water levels fell several meters below lake basin outlets during an early postglacial dry climate in the Holocene (younger than 10,000 radiocarbon years, or about 11,500 calibrated or calendar years before present (B.P.)). Water levels in the Huron basin, for example, fell more than 20 meters below the basin overflow outlet between about 7900 and 7500 radiocarbon (about 8770–8290 calibrated) years B.P. Outlet rivers, including the Niagara River, presently falling 99 meters from Lake Erie to Lake Ontario (and hence Niagara Falls), ran dry. This newly recognized phase of low lake levels in a dry climate provides a case study for evaluating the sensitivity of the Great Lakes to current and future climate change.

Evolution of lakes in the Huron basin: Deglaciation to present

Water bodies, ancestral to the present lakes including Lake Huron, first appeared in the southern Great Lakes basin about 15,500 14 C years (18,800 cal years) BP during the oscillatory northward retreat of the last (Laurentide) ice sheet from its maximum position south of the Great Lakes watershed. Glacial lakes, impounded by a retreating ice margin on their northern shores, were continuously present after 13,000 14 C (15,340 cal) BP for 3000 14 C (3900 cal) years. Drainage routings varied in time through the Erie and Michigan basins to the Mississippi River system, a probable source for colonizing aquatic organisms, then to the Ontario basin, and finally northeastward to the Ottawa River valley via the isostatically-depressed North Bay outlet by 10,000 14 C (11,470 cal) BP. Water levels were generally low between 10,000 and 7500 14 C (11,470 and 8300 cal) BP and may have risen several tens of metres for short periods due to overflow of meltwater from upstream subglacial reservoirs or from glacial lakes impounded by residual ice in the Hudson Bay watershed. About 8000 14 C (8890 cal) BP glacial runoff bypassed the Great Lakes, and Huron basin waters descended into hydrologic closure under the influence of the early Holocene dry climate. With increasing precipitation and water supply about 7500 14 C (8300 cal) BP the Huron water body again overflowed its North Bay outlet. Differential isostatic uplift (fastest to the north-northeast) raised this outlet and lake level relative to the rest of the basin. The lake finally overflowed southern outlets at Chicago and Port Huron-Sarnia by 5000 14 C (5760 cal) BP (during the Nipissing highstand). Enhanced erosion of the latter outlet and continued differential uplift of the basin led to the present configuration of Lake Huron and Georgian Bay.

Warmer and Drier Climates that Make Terminal Great Lakes

ABSTRACT A recent empirical model of glacial-isostatic uplift showed that the Huron and Michigan lake level fell tens of meters below the lowest possible outlet about 7,900 14C years BP when the upper Great Lakes became dependent for water supply on precipitation alone, as at present. The upper Great Lakes thus appear to have been impacted by severe dry climate that may have also affected the lower Great Lakes. While continuing paleoclimate studies are corroborating and quantifying this impacting climate and other evidence of terminal lakes, the Great Lakes Environmental Research Laboratory applied their Advanced Hydrologic Prediction System, modified to use dynamic lake areas, to explore the deviations from present temperatures and precipitation that would force the Great Lakes to become terminal (closed), i.e., for water levels to fall below outlet sills. We modeled the present lakes with pre-development natural outlet and water flow conditions, but considered the upper and lower Great Lakes separately with no river connection, as in the early Holocene basin configuration. By using systematic shifts in precipitation, temperature, and humidity relative to the present base climate, we identified candidate climates that result in terminal lakes. The lakes would close in the order: Erie, Superior, Michigan-Huron, and Ontario for increasingly drier and warmer climates. For a temperature rise of T°C and a precipitation drop of P% relative to the present base climate, conditions for complete lake closure range from 4.7T + P > 51 for Erie to 3.5T + P > 71 for Ontario.

Influence of loss of gradient from postglacial uplift on Red River flood hazard, Manitoba, Canada

The north-flowing, low-gradient section of the Red River in Manitoba has lost-60%/o of its valley gradient since 8 ka cal. BP. An existing hydraulic model of the modem Red River flood zone was used to examine the change in flood extent and depth of a discharge equivalent to the 1997 Red River flood (3970 m3/s) for scenarios of gradients at 8, 6, 4 and 2 ka cal. BP as well as 2 ka in the future. The modelling indicates a broad, shallow flood zone for all of the gradient scenarios, with extent and depth increasing over time. Between the 8 ka cal. BP and present-day scenarios, the flood zone increased from 1186 km2 to 1531 km2 (-29%/o) with depth increasing along four east-west cross-sections by 0.69 m (-61%), 0.91 m (-82%), 0.56 m (-64%) and 0.48 m (-86%). The flood extent and depths increased by a further 18 km2 (--5%) and 0.04-0.06 m (2-5%), respectively, by 2 ka in the future. Most of these changes to the flood zone occurred between 8 and 2 ka cal. BP, reflecting an exponential loss of gradient. A rise in flood depth equivalent to that which occurred between 8 ka cal. BP and the present-day, is assessed as increasing the long-term flood hazard; in contrast, the slight rise in depth between the present-day and 2 ka in the future does not.

Drumlins in Lake Ontario

Lake Ontario (74 m above sea level) is located 150–250 km north of the Late Wisconsinan southern limit of the Laurentide Ice Sheet in North America (Figure 1). Multibeam bathymetric mapping in deep eastern Lake Ontario (Figures 2, 3) revealed a set of parallel, straight, narrow ridges trending 235±5° which commonly rise 10–20 m above the surrounding lakefloor. The mapped ridges range in length to 6 km and from 60 to 600 m in width.

Paleolimnology of Lake Winnipeg

Brian J. Todd1, C. F. Michael Lewis2, L. Harvey Thorleifson3, Erik Nielsen4 & William M. Last5 1Geoterra Geoscience, 6 Shady Lane, Halifax, Nova Scotia, Canada B3N 1T9 (e-mail: bjtodd@fox.nstn.ca) 2Geological Survey of Canada (Atlantic), P.O. Box 1006, Dartmouth, Nova Scotia, Canada B2Y 4A2 (e-mail: mlewis@agc.bio.ns.ca) 3Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada K1A 0E8 (e-mail: thorleifson@gsc.nrcan.gc.ca) 4Manitoba Energy and Mines, 1395 Ellice Avenue, Winnipeg, Manitoba, Canada R3G 3P2 (e-mail: enielsen@em.gov.mb.ca) 5Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 (corresponding author; e-mail: WM Last@umanitoba.ca)

Diatom-inferred changes in effective moisture during the late Holocene from nearshore cores in the southeastern region of the Winnipeg River Drainage Basin (Canada)

The Winnipeg River Drainage Basin (WRDB), within the boreal forest region of northwest Ontario, is a region that is expected to be negatively affected by climate warming. Inferences of droughts over the past two millennia from Little Raleigh Lake were based on two nearshore sediment cores. The core locations were from depths of ~12 and 15 m and were based on sufficient nearshore sediment accumulation and distance from the modern benthic-to-planktonic diatom boundary, where a distinct shift from dominance of benthic taxa changed to dominance of planktonic taxa in surficial sediments at ~11.8 m. Diatom-inferred depth was based on a model developed from 60 surficial sediments within the study lake. Depth inferences indicate that prolonged periods of aridity occurred from ~ad 950 to 1300 (corresponds to ‘Medieval Climate Anomaly’) and from ~ad 1625 to 1750 (aridity during ‘Little Ice Age’). We found that the core collected from a depth closer to the benthic-to-planktonic diatom boundary was more sensitive to changes in lake level than the deeper core where planktonic diatoms dominated the assemblage. The inferred low-water stands of the past two millennia are well outside of the range of the past ~100 years, suggesting that recent drought history may not be a good estimate of future extremes.

Introduction to ''Holocene water levels and paleo-hydrology of the Laurentian Great Lakes''

The glacial Great Lakes in central eastern North America co-existed with the recession of the Laurentide Ice Sheet (LIS) from *14.0 to 9.5 (17.0 to 10.9 cal) ka BP as the ice margin receded from south of the Lake Erie basin (\41 N) to north of the Lake Superior basin ([49 N). Until recent decades the conventional view of the post-glacial Great Lakes was that of a long period of relatively stable water levels controlled by their outflow at basin outlet sills. In this collection of papers, evidence and understanding of the Holocene post-glacial Great Lakes are advanced mainly in terms of their responses to Holocene changes in climate, and to inflow and drainage from upstream water bodies still in contact with the residual LIS. Lake levels as a function of climate