CGRG Bibliography of Canadian Geomorphology
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Author : Friele, P.A.; Clague, J.J.; Simpson, K.; and Stasiuk, M.
Date : 2004.
Title : Holocene evolution of the Lillooet River delta: evidence for episodic sedimentation downstream from a Quaternary volcano.
Publication : International Conference on Sediment and Geochemical Budgets in Geomorphology to honour Professor Olav Slaymaker. June 27th - 30th, 2004 The University of British Columbia, Vancouver, Canada.
Issue :
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Abstract
The sediment fill in the Lillooet River valley records the post-glacial infilling of a ~75-km-long fjord lake downvalley from the Mount Meager volcanic complex, an area with high rates of instability and an episode of recent volcanism. Using a sediment budget approach, Jordan and Slaymaker (1991) and Slaymaker (1993) estimated the yield from active geomorphic processes in the Lillooet River watershed and attempted to reconcile their sum with the measured volume delivered to the delta front in the last century. Unable to account for about 50% of the observed historic yield, they concluded that the unexplained yield was associated with Little Ice Age paraglacial sedimentation, or underestimation of the frequency or magnitude of landslides from volcanic terrain. They proposed a modification of the paraglacial sediment concept (Church and Ryder 1972). In the revised model, Holocene sediment yield is episodic, with numerous pulses induced by volcanism, landslides, Neoglaciation, and land-use change. In 2002 a drilling program was conducted 32-65 km downstream from the volcano, providing an opportunity to test the sediment yield model proposed by Jordan and Slaymaker (1991). Data from 13 drill cores, 10-40 m deep, reveal a progradational alluvial sequence, deposited over the last 7000 years, consisting of river channel, bar and overbank sediments interlayered with volcaniclastic units deposited by debris and hyperconcentrated flows. The volcaniclastic sediments record at least four major sedimentation pulses associated with instability at Mount Meager. Over the last 7000 years, the average rates of floodplain aggradation and progradation were, respectively, 4.4 mm a-1 and 6 m a-1, but large landslide events have caused sediment pulsing. The long-term average progradation rate is similar to the pre-river training rate, but short-term rates, calculated over 50-year intervals that include documented sediment pulses, yield progradation rates of 100-150 m a-1. These values are four to six times higher than the highest recorded historical rates. The large sediment supply from Mount Meager biases the proportion of volcanic material carried as bedload by Lillooet River. Desloges and Church (1987), working in the Bella Coola River valley in the central Coast Mountains, found that the proportions of lithologies in channel gravels are similar to the proportions of bedrock types in the watershed. In contrast, volcanic bedrock underlies only 2% of the Lillooet River watershed, but volcanic lithologies comprise 25-75% of the gravel fraction of channel and hyperconcentrated flow deposits. These results demonstrate that the Mount Meager volcanic complex dominates sediment supply to the Lillooet River. Our work supports the pulse sedimentation model developed by Jordan and Slaymaker (1991) and indicates that the paraglacial sediment supply model (Church and Ryder 1972; Church and Slaymaker 1989) does not apply in settings downvalley of Quaternary volcanoes.
Bibliography of Canadian Geomorphology