Late Quaternary climatic influences on river geomorphology on the Alberta Plains, Canada
Abstract
The most obvious geomorphic aberrations on the flat Alberta plains, incised river valleys partly refilled with alluvium, are indirect products of changing climate in latest Pleistocene and early Holocene time. The valley bottoms lie 15 to 120 m below the general plains surface and cut through till-bedrock contacts, indicating that rivers established their present courses following deglaciation. Previous hypotheses for incision invoked post-glacial isostatic rebound, but rebound models show that base levels rose downstream during and after deglaciation, a situation not conducive to incision. We hypothesize that large quantities of meltwater from the retreating Cordilleran Ice Sheet generated rapid incision for a period of about 2 000 years following the retreat of the ice sheets (14-12ka.) In this study, a combined ice sheet-climate model is used to estimate the amount of water derived from the melting Cordilleran Ice Sheet between 14 and 12ka; resulting annual discharges allocated to each basin indicate that major rivers were approximately 3 times greater in discharge than their modern counterparts. Experiments with the bedrock equation suggest these discharges are capable of causing the dramatic incision of Alberta rivers. Uncertainty concerning the duration and magnitude of large floods operating during deglaciation creates large variations in results; however, even the most conservatively estimated discharges are shown to be capable of causing incision of rivers to depths greater than indicated by field observations. Very soon after incision, rivers on the Alberta plains began aggrading, and deposited fills up to 35 m thick. Radiocarbon ages of bone fragments indicate filling was in progress ca. 13-12 ka. Previous work on paraglacial sedimentation is suggestive of an indirect climate-change trigger for aggradation: debris-laden valley walls in the Canadian Rockies began shedding sediment into the major rivers as the valley became progressively more ice-free. Hence plains evolution in latest Pleistocene and early Holocene time was mainly a consequence of changes in sediment/water balance in rivers, generated by changing climate. During the Holocene the major rivers proceeded to episodically reincise into their own alluvial fills, leaving terraces behind. Holocene degradation may merely be a continuation of long-term downcutting, which resumed when major sediment sources dried up. But presently there are no robust hypotheses tying terrace development to climatic (or tectonic) drivers.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMEP41C0813M
- Keywords:
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- 1625 GLOBAL CHANGE Geomorphology and weathering