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Two distinct episodes of increased water flux imposed on the Great Lakes system by discharge from upstream proglacial lakes during the period from about 11.5 to 8 ka resulted in expanded outflows, raised lake levels and associated climate changes. The interpretation of these major hydrological and climatic effects, previously unrecognized, is mainly based on the evidence of former shorelines, radiocarbon-dated shallow-water sediment sequences, paleohydraulic estimates of discharge, and pollen diagrams of vegetation change within the basins of the present Lakes Superior, Michigan, Huron, Erie and Nipissing. The concept of inflow from glacial Lake Agassiz adjacent to the retreating Laurentide Ice Sheet about 11-10 and 9.5-8.5 ka is generally supported, with inflow possibly augmented during the second period by backflooding of discharge from glacial Lake Barlow-Ojibway.Although greater dating control is needed, six distinct phases can be recognized which characterize the hydrological history of the Upper Great Lakes from about 12 to 5 ka; 1)an early ice-dammed Kirkfield phase until 11.0 ka which drained directly to Ontario basin; 2) an ice-dammed Main Algonquin phase (11.0-10.5 ka) of relatively colder surface temperature with an associated climate reversal caused by greater water flux from glacial Lake Agassiz; 3) a short Post Algonquin phase (about 10.5-10.1 ka) encompassing ice retreat and drawdown of Lake Algonquin; 4) an Ottawa-Marquette low phase (about 10.1-9.6 ka) characterized by drainage via the then isostatically depressed Mattawa-Ottawa Valley and by reduction in Agassiz inflow by the Marquette glacial advance in Superior basin; 5) a Mattawa phase of high and variable levels (about 9.6-8.3 ka) which induced a second climatic cooling in the Upper Great Lakes area. Lakes of the Mattawa phase were supported by large inflows from both Lakes Agassiz and Barlow-Ojibway and were controlled by hydraulic resistance at a common outlet -the Rankin Constriction in Ottawa Valley -with an estimated base-flow discharge in the order of 200000 m3s-1 6) Lakes of the Nipissing phase (about 8.3-4.7 ka) existed below the base elevation of the previous Lake Mattawa, were nourished by local precipitation and runoff only, and drained by the classic North Bay outlet to Ottawa Valley.

On the basis of extensive sampling and echo sounding, three major lithological units are recognized in the main basin of Lake Huron: (1) glacial till and bedrock; (2) glaciolacustrine clay; and (3) postglacial mud. Sand is a lesser unit in the Huron surficial sediments. Owing to the wide range in bathymetric complexity, postglacial muds occur in basins of three distinct types:Type A. Regular basins in which mud forms a continuous cover.Type B. Irregular basins with undulating bottom topography. Glaciolacustrine clays outcrop in the crests and mud fill occurs in the hollows. Mud cover is greater than 50%.Type C. As for type B but with mud cover less than 50%.The sediment distribution pattern is essentially simple with a natural superposition of sediment units reflecting the glacial and postglacial history of the lake. A bedrock escarpment from Point Clark to Thunder Bay divides Lake Huron into two distinct morphological regions. To the south of the escarpment, the lake has a gentle topography. A second low amplitude escarpment, trending northeast from Ipperwash, divides the southern region of the lake into two large depositional basins. To the north of the major escarpment, the lake is much deeper and has a complex bottom topography. The northern region is separated into two large basins of discontinuous sediment deposition by a rise of glacial sediments trending north from Thunder Bay. The inshore region of Lake Huron and the two escarpments are composed of glacial till and bedrock. The till is overstepped in the deeper water by glaciolacustrine clays which are themselves overstepped by postglacial muds. Postglacial mud accumulation is continuous in the southern basins due to the gentle relief of the lake bottom. In the northern region of the lake, mud accumulation is discontinuous due to the undulating nature of the lake bottom. Mud fills the hollows leaving glaciolacustrine clay exposed at the top of the undulations in this region.The surficial sediments contain variable amounts of quartz, clay minerals, organic carbon, and carbonates. Quartz content is greatest in the coarser inshore sediments while clay minerals and organic carbon are greatest in the liner offshore sediments. Carbonate is low throughout the lake, except along the eastern edge. Blite is the dominant clay mineral with lesser amounts of chlorite and kaolinite.Amphipods, oligochaetes, and chironomids are the major benthic organisms in the Huron sediments. Amphipods are most numerous at a water depth of 70 m, oligochaetes at 140 m, and chironomids in the shallowest depths.The textural characteristics of the sediments, defined by moment measures, can be interpreted as resulting from variable mixing of a clay and a sand end member population.Both end member populations are leptokurtic and asymmetrically skewed due to the introduction of a silt size mode predominantly in the form of a carbonate. The sand end member population is positively skewed and occurs in the inshore zone comprising the coastal nearshore region and the shallow mid-lake regions. The clay end member is negatively skewed and occurs in the depositional basins. Between these two extremes there is a gradual prograding from sand to clay related to a progressive mixing of the two populations. This mixing is believed to be a direct function of declining energy with increasing water depth.Sediment composition reflects both the source materials and the textural properties. The sediments of the southern basin are derived predominantly from glacial tills whereas the composition of the sediments of the northern basin has been substantially modified by dilution with reworked glaciolacustrine clays.

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Seismic reflection profiling and piston coring identified seismic reflectors in northern Lake Huron and Georgian Bay linked with unconformities caused by at least six reductions in lake level. In ascending stratigraphic order, these lowstands occurred at about 11 200 BP, associated with the Kirkfield outlet from early Lake Algonquin; 10 200 – 9900 BP, associated with the post-Algonquin lake level fall; 9800 – 9050 BP, the most extreme lowstand, associated with the main Stanley – Hough draw down; and 7800 – 7450 BP. The concomitant highstands are Lake Algonquin, from about 11 200 – 10 200 BP; early Lake Mattawa, between 9600 and 9350 BP; the main Mattawa phase, 9050–7800 BP; and the Nipissing highstand, at about 4700 BP. Isotopic and paleoecological data show that all of the lowstands are characterized by cold, dilute, and isotopically very light (< −20‰) waters from the melting Laurentian ice cap. Highstands, on the other hand, are characterized by higher dissolved solid concentrations and a much smaller meltwater component. Oxygen isotope values of the waters in these lakes were −15 to −17‰ in Lake Algonquin, −13 to −14‰ for the early and middle Mattawa stages, −9 to −8‰ for the main Mattawa stage, and −7‰ for modern waters. This association of lowstands and not highstands with isotopically light waters is a new contribution to early Holocene hydrology of the Great Lakes. The Younger Dryas cool episode is coeval with the Lake Algonquin highstand and a younger cool episode is generally coeval with the Mattawa highstand. This supports the hypothesis of C.F.M. Lewis and T.W. Anderson that these large cold lakes were responsible for regional cooling during the early Holocene.

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