Postglacial sediment budget of Chilliwack Valley, British Columbia Jon Tunnicliffe, 1 * Michael Church, 1 John J. Clague 2 and James K. Feathers 3 1 Department of Geography, The University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z2 2 Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6 3 Department of Anthropology, University of Washington, Seattle, Washington, USA. 98195-3100 Received 15 August 2011; Revised 8 February 2012; Accepted 16 February 2012 *Correspondence to: J. Tunnicliffe, Department of Geography and Environmental Studies, Carleton University, 1125 Colonel By Drive, Ottawa, ON, Canada, K1S 5B6. E-mail: jon_tunnicliffe@carleton.ca 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 km 2 ) 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 km 3 of glacial material has been evacuated from the valley. Copyright © 2012 John Wiley & Sons, Ltd. KEYWORDS: sediment budget; paraglacial; optically stimulated luminescence; Holocene; Chilliwack Valley, British Columbia Introduction It has long been recognized that, within large mountain drainage basins, the process of relaxation from the radical sedimentary disturbance of glaciation lasts for many thousands of years (c.f. the paraglacial concept of Church and Ryder, 1972). The timing and nature of postglacial relaxation is closely tied to drainage basin scale: the chronology of sediment transfer is generally different in alpine headwaters, where hillslope processes have largely settled into equilibrium, than in larger mainstem valleys (>1000 km 2 ) where glaciolacustrine and glaciofluvial sediments continue to be eroded and redeposited (Church and Slaymaker, 1989). In glaciated valleys, prominently dissected valley-fill deposits and blankets or veneers of till on the hillslopes are ubiquitous features that provide additional sediment load to the river along its course. River grade and morphological style reflect the enduring influence of till and of glaciolacustrine and glaciofluvial valley fills. Quantitative studies spanning a range of temporal and spatial scales have sought to characterize changes in hillslope and fluvial sediment flux in the interval between ice retreat and the asymptotic return to postglacial equilibrium. Sediment budget studies range in catchment scale from less than 50 km 2 to greater than 1000 km 2 . The former have looked at the retreat of glacier ice and its effect on alpine river and hillslope systems (Warburton, 1990; Ballantyne and Benn, 1994; Gurnell et al., 1999; Orwin and Smart, 2004; Meigs et al., 2006) while the latter have documented the chronologies of river valley evolu- tion based on available evidence from terraces, fans, deltas and lake fills (Jackson et al., 1982; Brooks, 1994; Müller, 1999; Hinderer, 2001; Dirszowsky and Desloges, 2004). A principal finding is that the process of paraglacial reworking of glacial sediments can be relatively rapid (<100 years) in proximal settings, while the response at more distal points in the catchment network tends to be complex and extended in time. It requires good volumetric and chronologic control to interpret the postglacial sediment budget. Studies have empha- sized the problems of interpreting the historical development of valley-bottom landforms when little is known about system storage, network connectivity (e.g. from lateral hillslope sources to tributaries, thence to trunk streams) and disturbances that act at varying scale and intensity over the course of the postglacial period (Jordan and Slaymaker, 1991; Church, 2002; Friele and Clague, 2009; Otto et al., 2009). Sediment budget studies in larger catchments show clearly that the EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms 37, 1243–1262 (2012) Copyright © 2012 John Wiley & Sons, Ltd. Published online 27 March 2012 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/esp.3229