Rapid and large-scale landscape modification caused by the draining of a glacier-dammed lake in British Columbia, Canada

Abstract

River terraces are ubiquitous in the formerly glaciated valleys of British Columbia, where they are commonly interpreted as products of Holocene incision of paraglacial valley fills. In this paper, we describe and interpret terrace sequences on the former floor of glacial Lake Fraser, the largest Late Pleistocene glacier-dammed lake in British Columbia. Postglacial terraces up to 120 m above the present lower Nechako and Fraser River valley floors are the dominant landforms within the former lake basin. In this paper, we focus on the Nechako River terraces between Stuart River and Prince George, a distance of about 85 km. Our reconstruction and analysis are based on high-resolution lidar imagery and field investigation of sediments exposed beneath the terraces. The highest, unpaired terraces are mainly erosional landforms developed on glacial Lake Fraser glaciolacustrine sediments, but are ornamented with low biconvex flood bars and linear erosional channels. As water drained rapidly eastward out of the lake, thin flows of fluidized lacustrine silt and sand derived from adjacent valley walls were deposited on the highest terraces and flood dunes formed below the lowest paleo-lake shorelines. At the same time, huge amounts of sediment were stripped from the emergent valley walls by landslides and gullying. This large and rapid flux of sediment provided thick gravel and sand that underlie lower Nechako Valley terraces. The lowest terraces have meandering channel forms that are markedly different from the streamlined high terrace surfaces. About 200 kettles in six clusters on mid- and low-level terraces in Nechako Valley mark locations of ice blocks that calved from the stagnating front of the remnant Cordilleran Ice Sheet to the west and became stranded on the floor of glacial Lake Fraser. The distribution of the kettles shows that Nechako and, by inference, Fraser rivers were flowing within a few metres of their present levels within years to several decades after glacial Lake Fraser drained, which challenges the common assumption implicit in the paraglacial model that the terraces developed more slowly throughout the Holocene.