Controls on North American runoff magnitude and routing

Runoff from the melting Laurentide Ice Sheet (LIS) and precipitation, plus abrupt releases of stored water in proglacial lakes along the southern margin of the LIS, discharged at different times to the Gulf of Mexico, North Atlantic Ocean, western Arctic Ocean and finally to Hudson Bay. A primary control on the variability in runoff routing stems from the changing configuration of hydrological catchments and drainage basins during deglaciation, which developed by the combination of location of the ice margin, relief on the ice sheet and the deglaciated continental surface, and differential isostatic rebound. The latter raised the land surface more in the northern and northeastern regions, and shifted overflow from proglacial lakes from one outlet to another.

In this chapter, the four main glacial drainage basins (Fig. 28.2) that directed runoff to widely separated continental margins will be discussed. These basins are:

1 the Mississippi River Valley (draining to the Gulf of Mexico);

2 the St Lawrence and Hudson River valleys (draining to the North Atlantic Ocean);

Figure 28.1 Map of North America and Greenland showing four ice sheets at their maximum extent in the last glaciation. Oscillations in the various routings of precipitation and ice-melt runoff from the southern part of the Laurentide Ice Sheet between 18 ka and 7 ka and their oceanic impact are the subject of this chapter. (See www.blackwellpublishing.com/knight for colour version.)

Laurentide Ice Sheet

Figure 28.1 Map of North America and Greenland showing four ice sheets at their maximum extent in the last glaciation. Oscillations in the various routings of precipitation and ice-melt runoff from the southern part of the Laurentide Ice Sheet between 18 ka and 7 ka and their oceanic impact are the subject of this chapter. (See www.blackwellpublishing.com/knight for colour version.)

3 the Athabasca-Mackenzie River Valley (draining to the Beaufort Sea and western Arctic Ocean);

4 Hudson Bay watershed (draining via Hudson Strait to the western Labrador Sea and the northern North Atlantic Ocean).

Growth of the LIS blocked the northward-draining basins. This blockage, together with the great elevation of ice, ranging up to 2-3km over Hudson Bay and adjacent regions, drastically changed the configuration of continental surface slopes. In addition, the Earth's crust was isostatically depressed hundreds of metres by the ice load (Peltier, 1998). The surface configurations of the ice sheet and the continental surface beyond the ice margins defined new 'cryohydrologicaT drainage basins for continental runoff (Teller, 1990a,b; Licciardi et al, 1999). At the LIS maximum, about 21-18ka (25,000-21,000cal.yr), all drainage

Figure 28.2 A similar view as in Fig. 28.1 showing four continental watersheds which played significant roles in directing runoff to the oceans. These watersheds are the Mississippi Valley catchment draining to the Gulf of Mexico, the Great Lakes-St Lawrence Valley catchment discharging to the North Atlantic Ocean, the Athabasca-Mackenzie Valley catchment draining to the western Arctic Ocean, and the Hudson Bay catchment discharging via Hudson Strait to the northern Atlantic Ocean (western Labrador Sea). Outlets that were significant controls on runoff routing include Chicago (A) from southern Lake Michigan basin, Wabash Valley (B) from western Lake Erie basin, Mohawk Valley (C) connecting Lake Ontario basin with Hudson River Valley, Hudson River Valley (D) draining to Long Island Sound and the Atlantic Ocean, Hudson Shelf Valley (E), a possible extension of the Hudson Valley route (R. Thieler, personal communication, 2004), southern Lake Agassiz outlet (F), first eastern Agassiz outlet (G), northwestern Agassiz outlet (H), second eastern Agassiz outlet (I), and Kinojevis outlet (J) to Ottawa and St Lawrence River valleys. See Fig. 28.4a for names and locations of Great Lakes basins. (See www.blackwellpublishing.com/knight for colour version.)

Figure 28.2 A similar view as in Fig. 28.1 showing four continental watersheds which played significant roles in directing runoff to the oceans. These watersheds are the Mississippi Valley catchment draining to the Gulf of Mexico, the Great Lakes-St Lawrence Valley catchment discharging to the North Atlantic Ocean, the Athabasca-Mackenzie Valley catchment draining to the western Arctic Ocean, and the Hudson Bay catchment discharging via Hudson Strait to the northern Atlantic Ocean (western Labrador Sea). Outlets that were significant controls on runoff routing include Chicago (A) from southern Lake Michigan basin, Wabash Valley (B) from western Lake Erie basin, Mohawk Valley (C) connecting Lake Ontario basin with Hudson River Valley, Hudson River Valley (D) draining to Long Island Sound and the Atlantic Ocean, Hudson Shelf Valley (E), a possible extension of the Hudson Valley route (R. Thieler, personal communication, 2004), southern Lake Agassiz outlet (F), first eastern Agassiz outlet (G), northwestern Agassiz outlet (H), second eastern Agassiz outlet (I), and Kinojevis outlet (J) to Ottawa and St Lawrence River valleys. See Fig. 28.4a for names and locations of Great Lakes basins. (See www.blackwellpublishing.com/knight for colour version.)

from the southern portion of the LIS was directed south to the Gulf of Mexico or east to the Atlantic Ocean (Figs 28.1 & 28.2). For the next 13-10 kyr (16,500-12,500cal.yr ago), watershed configurations were dynamic, controlled by shifting ice divides as the LIS retreated and downwasted. The routing of runoff repeatedly changed, at times abruptly, whenever the retreating ice margin uncovered alternative lower drainage pathways to different sectors of the continental margin (Licciardi et al., 1999).

The routing of runoff was also influenced by isostatic recovery of the crust, as ice thinned and retreated. The amplitude of recovery was greatest where ice loads had been thickest. Although some recovery was instantaneous upon reduction of the ice load (elastic crustal response), much uplift was influenced by redistribution of viscous mantle material beneath the lithosphere, which caused crustal recovery or vertical uplift to proceed on a millennial time-scale, generally at a decaying exponential rate (Peltier, 1998). Recovery amplitudes diminished toward the outer limits of the former ice sheet, and this effect imparted a differential aspect to the isostatic recovery. As a result, former horizontal surfaces, for example glacial-lake shorelines, are now uplifted more in the north than in the south (e.g. Teller & Thorleifson, 1983; Larsen, 1987; Lewis & Anderson, 1989).

Precipitation runoff and melting of the ice sheet during deglaciation yielded annual baseline flows of freshwater to the adjacent oceans that significantly exceeded postglacial discharges (Teller, 1990a,b; Licciardi et al., 1999) (Fig. 28.3a). An important increment to the baseline flow arose from the repeated rapid drawdown of lakes that were impounded between the downslope ice margin and the upslope continental surface (Teller et al., 2002; Teller & Leverington, 2004) (Fig. 28.3a). These outbursts channelled large volumes of water through newly opened outlets for short periods, and were superposed on the baseline flow of precipitation and meltwater runoff (see Teller & Leverington, 2004, fig. 5). Outlet erosion, especially during catastrophic outbursts from lakes when large flows entrenched spillways, also affected the subsequent routing of runoff from the lakes.

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