Postglacial sediment budget of Chilliwack Valley, British Columbia

Abstract

The paraglacial reworking of glacial sediments by rivers and mass wasting is an important conditioning factor for modern sediment yields in mountainous catchments in formerly glaciated regions. Catchment scale and patterns of sediment storage are important influences in the rate of postglacial adjustment. We develop a quantitative framework to estimate the volume, sediment type, and fractional size distribution of legacy glacial materials in a large (1230 km2) watershed in the North Cascade Mountains in south‐western British Columbia, Canada. Chilliwack Valley is exceptional because of the well‐dated bounds of deglaciation. Interpolation of paleo‐surfaces from partially eroded deposits in the valley allows us to estimate the total evacuated sediment volume. We present a chronology of sediment evacuation from the valley and deposition in the outlet fan, based on infrared stimulated luminescence (IRSL) and 14 C dating of river terraces and fan strata, respectively. The effects of paraglacial sedimentation in Chilliwack Valley were intensified through a major fall in valley base‐level following ice retreat. The steepened mainstem valley gradient led to deep incision of valley fills and fan deposits in the lower valley network. The results of this integrated study provide a postglacial chronology and detailed sediment budget, accounting for long‐term sorting of the original sediments, lag deposit formation in the mainstem, deposition in the outlet fan, and approximate downstream losses of suspended sediment and wash load. The mass balance indicates that a bulk volume of approximately 3.2 km3 of glacial material has been evacuated from the valley. Copyright © 2012 John Wiley & Sons, Ltd.

The paraglacial geomorphology of the Fraser Lowland, southwest British Columbia and northwest Washington

“…6A), and its shorelines are evident on both sides of the border. The paraglacial Chilliwack alluvial fan (Tunnicliffe et al, 2012) intersects the drained Sumas lake shoreline deposits at its western end. The lake was drained in the 1920s (Environment Canada, 1992).…”

Vegetation succession in deglaciated landscapes: implications for sediment and landscape stability

“…Holocene or Little Ice Age; Passmore and Waddington, 2009;Hobley et al, 2010) timescales and are often limited in spatial area (e.g. 5-1200 km 2 ; Irvine-Fynn et al, 2011;Tunnicliffe et al, 2012). Given the increasing recognition of vegetation-landscape interactions, and previous difficulties in studying intermediate timescales of paraglacial adjustment, it is likely that the importance and role of these interactions in determining the timescale and processes of paraglacial adjustment is lacking.…”

A 1‐D morphodynamic model of postglacial valley incision

“…The initial conditions for the model are set to a time when Fraser ice had retreated from Vedder Crossing, dropping the valley base level, and the river began to resculpt its longitudinal profile [ Tunnicliffe et al ., ]. There is some uncertainty as to the bounding topography of the lower valley deposits at this time; several different scenarios were run to assess the effect of this uncertainty on the computed flux rates, response time.…”

“…This top node delivers water and sediment starting at relatively high rates, tapering off over the course of each model run, in accordance with our understanding of paraglacial sediment contributions from tributaries in the early Holocene. Falling base level within the tributaries leads to the evacuation of large stores of fluvial and glaciofluvial material [e.g., Brooks , ; see Tunnicliffe et al ., ]. The downstream boundary is mostly depositional, with coarse‐textured material building up the profile of Vedder Fan.…”

“…Chilliwack River underwent major postglacial degradation following the retreat of the Fraser ice lobe (see summary in Tunnicliffe et al . []) at roughly 13,000 calendar years B.P. The headward migration of incision was halted at a bedrock knickpoint roughly 30 km upstream of the valley outlet where the modern river winds around an outcrop of Permian phyllite.…”