Paleotopographic reconstructions of the eastern outlets of glacial Lake Agassiz

Paleotopographic reconstructions of the eastern outlets of glacial Lake Agassiz provide a foundation for understanding the complex manner in which terrain morphology controlled the routing of overflow through the eastern outlets during the lake's Nipigon Phase (ca. 9400-8000 ''^C years BP) and for understanding the causes of outlet-driven declines in lake level during that period. Although flow paths across the divide between the Agassiz and Nipigon basins were numerous, eastward releases from Lake Agassiz to glacial Lake Kelvin (modern Lake Nipigon) were channeled downslope into a relatively small number of major channels that included the valleys of modern Kopka River, Ottertooth Creek, Vale Creek, Whitesand River, Pikitigushi River, and Little Jackfish Riven From Lake Kelvin, these waters overflowed into the Superior basin. The numerous lowerings in lake level between stages of the Nipigon Phase, controlled by topography and the position of the retreating southern margin of the Laurentide Ice Sheet, had magnitudes of between 8 and 58 m, although some of these drawdowns may have occurred as multiple individual events that could have been as small as several metres. The total volumes of water released in association with these drops were as great as 8100 km^, and for all Nipigon stages were probably between about 140 and 250 km^ per metre of lowering. The topographic reconstructions demonstrate that Lake Agassiz occupied the area of glacial Lake Nakina (located northeast of modern Lake Nipigon) by the The Pas stage (ca. 8000 '*C years BP) and that Lake Agassiz drainage through the Nipigon basin to the Great Lakes ended before the succeeding Gimli stage. Resume : Des reconstructions paléotopographiques des chenaux du côté est du lac glaciaire Agassiz fournissent une base pour comprendre la façon complexe selon laquelle la morphologie de terrain contrôlait le cheminement du débordement par les déversoirs vers l'est durant la phase Nipigon du lac (9400-8000 années '"^C avant le présent) et pour comprendre les causes des baisses du niveau de lac causées par les déversoirs durant cette période. Même s'il existait de nombreux chemins d'écoulement traversant la ligne de partage des eaux entre les bassins d'Agassiz et de Nipigon, l'eau sortant à l'est du lac Agassiz vers le lac glaciaire Kelvin (le lac Nipigon actuel) était canalisée vers le bas de la pente dans un nombre relativement restreint de chenaux majeurs qui comprenaient les vallées actuelles de la rivière Kopka, du ruisseau Ottertooth, du ruisseau Vale, de la rivière Whitesand, de la rivière Pikitigushi et de la rivière Little Jackfish. À partir du lac Kelvin, ces eaux se déversaient dans le bassin du Supérieur. Les nombreux abaissements de niveau de lac entre les étages de la phase Nipigon, contrôlés par la topographie et la position de la bordure sud, en retrait, de l'inlandsis Laurentien, avaient des amplitudes de 8 à 58 m, bien que quelques-uns de ces abaissements aient pu être des événements multiples individuels de quelques mètres seulement. Les volumes totaux d'eau relâchée, associés à ces abaissements, ont atteint jusqu'à 8100 km^ et, pour tous les étages Nipigon, les abaissements étaient probablement de 140 à 250 km^ par mètre d'abaissement. Les reconstructions topographiques démontrent que le lac Agassiz occupait la région du lac glaciaire Nakina (situé au nord-est du présent lac Nipigon) à l'époque de l'étage The Pas (vers 8000 années '*C avant le présent) et que le drainage du lac Agassiz à travers le bassin de Nipigon vers les Grands Lacs a cessé avant l'étage suivant de Gimli. [Traduit par la Rédaction] Introduction decreased in elevation toward the north; thus, as the southern margin of the Laurentide Ice Sheet retreated northward during During the Nipigon Phase of glacial Lake Agassiz (ca. the Nipigon Phase, progressively lower outlets were opened 9400-8000 '"^C years BP; 10400 8800 cal years BP), overflow to glacial Lake Kelvin (modern Lake Nipigon), and, correwas routed into the Nipigon basin through a complex system spondingly, the level of Lake Agassiz declined, known collectively as the eastern outlets (Elson 1957, 1967; Field studies have provided the basis for understanding Zoltai 1965a, 1967; Teller and Thorleifson 1983, 1987). These the nature of the eastern outlets and their role in the late outlets consisted of a series of eastward flow paths that Quaternary history of central North America (e.g., Zoltai Received 16 January 2003. Accepted 15 May 2003. Published on the NRC Research Press Web site at http://cjes.nrc.ca on 25 September 2003. Paper handled by Associate Editor R. Gilbert. D.W. Leverington.^ Center for Earth and Planetary Studies, Smithsonian Institution, Washington, DC 20560-0315, U.S.A. J.T. Teller. Department of Geological Sciences, University of Manitoba, Winnipeg, MB R3T 2N2, Canada. 'Corresponding author (e-mail: [email protected]). Can. J. Earth Sei. 40: 1259-1278 (2003) doi: 10.n39/E03-043 © 2003 NRC Canada 1260 Can. J. Earth Sei. Vol. 40, 2003 1965a, 1965^, 1967; Thorleifson 1983; Teller and Thorleifson 1983, 1987; Teller and Mahnic 1988; Lemoine and Teller 1995). Interpretations have been hindered, however, by the paucity of preserved Lake Agassiz shoreline features in the region around the divide between the Agassiz and Nipigon basins, the generally low topographic relief along the divide, the lack of easy ground access to the region, and the absence of detailed topographic data. The purpose of this research is to generate and interpret high-resolution digital topographic models of the eastern outlet region for all recognized stages of the Nipigon Phase of Lake Agassiz, to increase understanding of the eastern outlet system, and to provide an improved framework for guiding future field-based investigations. For each investigated stage, the corresponding topographic reconstruction was used to (/) approximate the position and configuration of the Lake Agassiz shoreline in the vicinity of the eastern outlets, (//) determine the position and morphology of the drainage divide separating the Lake Agassiz basin from the Nipigon basin, (///) identify major drainage routes from Lake Agassiz to Lake Kelvin, and (iv) further quantify aspects of the overflow and outburst history of Lake Agassiz. Lake Agassiz overview During déglaciation, the retreating southern margin of the Laurentide Ice Sheet (LIS) gradually exposed large expanses of the North American continent, including the structural and topographic basin of north-central North America, the central part of which today contains the waters of Hudson Bay (Teller 1987). Located within this basin, the LIS acted to impede northward drainage, at times causing waters draining from surrounding regions to pool against the ice, forming proglacial lakes. Although many of these lakes were relatively small and short-lived, the largest. Lake Agassiz, was a major feature of late-glacial North America for most of its 5000 calendar year existence (e.g., Upham 1895; Elson 1967; Teller 1987) (Fig. 1). Glacio-isostatic rebound and changing ice-sheet configurations caused the size of Lake Agassiz to vary considerably during its history (e.g., Elson 1967; Teller 1985; Leverington et al. 2000, 2002a; Teller and Leverington, in preparation^), and numerous catastrophic releases of water occurred when lake levels dropped after lower outlets were deglaciated (e.g.. Teller and Thorleifson 1983; Teller 1985; Leverington et al. 2002a). Lake Agassiz overflows and outbursts were important influences on the rivers and lakes that received them (e.g., Clayton 1983; Teller and Thorleifson 1983; Teller 1985, 1987, 1990a; Teller and Mahnic 1988; Lewis et al. 1994), and both outbursts and major reroutings of overflow from Lake Agassiz may have influenced the North Atlantic ocean-climate system (e.g., Broecker et al. 1989; Teller 1990fo; Barber et al. 1999; Clark et al. 2001; Teller et al. 2002; Fisher et al. 2002). The history of Lake Agassiz, which extended from about 11 700 to 7700 '''^C years BP, has been divided into five major phases: Lockhart, Moorhead, Emerson, Nipigon, and Ojibway (Fenton et al. 1983; Teller and Thorleifson 1983) (Fig. 2). During the Lockhart Phase (about 11 700 to 10 800 "^C years BP) (Fenton et al. 1983; Fisher 2003), drainage was through the lake's southern outlet to the Gulf of Mexico, via the Minnesota and Mississippi river valleys (Elson 1967) (Fig. 1, outlet S). The Lockhart Phase was terminated when the Kaministikwia route to Lake Superior was deglaciated, causing a rapid drop in lake level and the abandonment of the southern outlet (e.g., Fenton et al. 1983; Teller and Thorleifson 1983) (Fig. 1, outlet K). During the Moorhead Phase, Lake Agassiz gradually expanded and transgressed southward due to a combination of differential glacio-isostatic rebound and a readvance of the LIS across drainage routes to the east (Elson 1967; Teller and Thorleifson 1983; Thorleifson 1996; Teller 2002). By the end of the Moorhead Phase, the southward-transgressing waters of Lake Agassiz reached the southern outlet for a brief time (Fig. 2), and outflow was once again south to the Gulf of Mexico (Thorleifson 1996; Fisher and Souch 1998; Teller 2001). During the Emerson Phase (about 10 100 to 9400 '"^C years BP), Lake Agassiz drainage was mainly through the deglaciated northwestern outlet (the Clearwater Spillway) to the Arctic Ocean via the Mackenzie River valley (Smith and Fisher 1993; Fisher and Smith 1994; Fisher and Souch 1998; Thorleifson 1996; Teller 2001) (Fig. 1, outlet NW; Fig. 2). By about 9400 '''^C years BP, both the southern and northwestern outlets had been abandoned (Fig. 2), and for the next -1400 '"*C years drainage was east through progressively lower and more northerly outlets to the Nipigon basin (defining the Nipigon Phase) (Elson 1967; Zoltai 1967; Thorleifson 1983, 1996; Teller and Thorleifson 1983; Fisher 2003) (Fig. 1, outlet E). Lake Agassiz gradually shifted northward during this phase, as it followed the receding southern margin of the LIS. At the beginning of the Ojibway Phase (about 8000 "^C years BP), Lake Agassiz merged with glacial Lake Ojibway to the east (e.g.. Hardy 1977; Vincent and Hardy 1979; Teller and Thorleifson 1983; Veillette 1994), forming a water body that extended from northern Manitoba to western Quebec. Associated with this merger was the abandonment of the outlets to the Nipigon basin, and the southeastward drainage of lake waters through the Angliers outlet (and, later, the nearby Kinojévis outlet) to the Ottawa River (Hardy 1977; Vincent and Hardy 1977, 1979; Veillette 1994) (Fig. 1, outlet KIN). By the end of the Ojibway Phase, Lake Agassiz-Ojibway extended as much as 1500 km along the southern margin of the LIS. At about 7700 '''^C years BP (Barber et al. 1999), the stagnant remains of the confining ice margin were breached, causing the lake to catastrophically drain northward into the Tyrrell Sea (Hudson Bay) (Hardy 1977; Vincent and Hardy 1979; Klassen 1983; Dredge 1983; Veillette 1994; Barber et al. 1999; Leverington et al. 2002a; Teller et al. 2002; Clarke et al. 2003) (Fig. 1, HB). By the end of the 5000 calendar year existence of Lake Agassiz, its waters had covered a total surface area of 1.5 million km^. Tlie eastern outiets to thie Nipigon basin The location and form of routes of overflow from Lake Agassiz were primarily controlled by two factors: (/) the position of the LIS; and (ii) terrain morphology, which was ^ Glacial Lake Agassiz: A 5000-year history of change and its relationship to the isotopic record of Greenland. In Preparation.