ABSTRACT: Comment on “Alternative routing of Lake Agassiz overflow during the Younger Dryas: New dates, paleotopography, and a re-evaluation” by Teller et al. (2005).

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The Teller et al. (2005) paper is of great interest to us and we agree that there is uncertainty about the chronology of hypothesized eastern outlet drainage (Fisher and Lowell, 2005; Fisher et al., 2005a and Fisher et al., 2005b; Lowell et al., in press) and thus its potential role in abrupt climate change. First, we note a resurgence of interest in this topic. Wally Broecker, seeking to understand the nature of the field evidence upon which the pattern of meltwater routing is developed, suggested two trips: to the eastern, and then to the other Lake Agassiz outlets. Flights over the southern outlet on the second trip revealed strandlines merging with a well-developed spillway, and in northern Alberta and Saskatchewan (the northwestern outlet), strandlines and boulder gravels lie at the head and within the large Clearwater–Athabasca spillway. As noted by Teller et al. (2005), no such field evidence was found along a suggested pathway for an eastern outlet (K on Fig. 1 of Teller et al., 2005). These preliminary observations made it evident that Upham's (1895) ‘probable hypothesis’, that water drained from Lake Agassiz eastward into the Superior Basin, had not been adequately tested prior to publication of several papers reconstructing the drainage routing across this region (e.g., Licciardi et al., 1999; Mann et al., 1999; Leverington et al., 2000; Leverington and Teller, 2003; Teller and Leverington, 2004). To clarify the evolving status on this topic, we comment on the three main points of Teller et al. (2005): (1) the age of deglaciation in the Thunder Bay region; (2) events in the Fort McMurray region; and (3) the possibility of readvances in the Thunder Bay and Fort McMurray regions. Documenting relic drainage pathways is challenging. Dates obtained by coring outlet spillway lakes provide age estimates for spillway abandonment (e.g., Fisher, 2003b), but age estimates for spillway initiation are dependent upon knowing the ice margin chronology. To be sure, the water level history in the main Lake Agassiz basin is important to the history of Lake Agassiz, and reconstructions require careful geomorphic, stratigraphic and chronologic analysis. The data from Teller et al. (2005) of six cores and one greater-than radiocarbon date adds little to our understanding of drainage from glacial Lake Agassiz. As pointed out by Karrow (2002), the Lake Agassiz basin is large, the workers few and the reconstructions increasingly complex. Though the drainage of glacial Lake Agassiz has been suggested as a major factor, even a trigger, of abrupt climate change, careful, critical analysis of outlet history and meltwater delivery is required before such a connection can be established.

Bibliography of Canadian Geomorphology