Diversions of glacial runoff from one entry point to the ocean to another resulted in freshening of surface seawater in widely separated regions. The history of the glacial palaeohydrology of North America is closely linked to the history of ice-marginal lakes. As will be discussed later, these changes are thought to be of great significance in influencing THC and climate. The following simplified account of the history of baseline deglacial runoff is tied to changes in proglacial lakes, and is structured to identify and characterize diversions of continental-scale runoff. Other, more detailed discussions of the history of North American glacial drainage and proglacial lakes can be found in publications such as Teller & Clayton (1983), Teller (1987, 1990a, 2004), Karrow & Calkin (1985), Fulton (1989), Teller & Kehew (1994) and Licciardi et al. (1999), as well as in many ofthe references cited in this chapter. In this account, the baseline runoffwas periodically augmented by large discharges arising from lake outburst floods when ice retreat opened new lower outlets from the glacial Great Lakes between about 16.5 and 11.2ka (19,700-13,100 cal.yr) and from glacial Lake Agassiz between 10.9 and 7.7ka (12,900 and 8450cal.yr) (Licciardi et al., 1999; Teller et al., 2002; Teller & Leverington, 2004) (Fig. 28.3). Releases of subglacial meltwater (Shaw, 1989, 2002; Flower et al., 2004), the timing and magnitude of which are poorly known, would have supplemented the surface runoff described in this chapter.

From the LGM to the Erie Interstade about 16.5ka (19,700 cal. yr), drainage from the entire southern margin of the LIS between the Appalachian (east) and the Cordilleran (west) mountains flowed to the Gulf of Mexico as baseline runoff (Figs 28.1 & 28.4a). This runoff would have consisted of large baseline flows of annual precipitation and meltwater modulated by seasonal variation. Runoff from eastern and northern sectors of the LIS would have gone to the North Atlantic and Arctic Oceans, respectively.

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