A method to assess fluvial fan channel networks, with a preliminary application to fans in coastal British Columbia Thomas Hugh Millard a, ⁎, Dan L. Hogan b , David J. Wilford c , Brian Roberts a a B.C. Ministry of Forests and Range, Coast Forest Region, Nanaimo, BC, Canada b B.C. Ministry of Forests and Range, Research Branch, Victoria, BC, Canada c B.C. Ministry of Forests and Range, Northern Interior Region, Smithers, BC, Canada abstract article info Article history: Received 8 January 2008 Received in revised form 16 May 2008 Accepted 15 June 2009 Available online 17 July 2009 Keywords: Alluvial fan Channel network Fans with forests Vancouver Island British Columbia The channel network on a fluvial fan distributes sediment across the fan surface and determines fan development. We present a method to characterize fan channel networks (FCNs) so that the effect of controls (e.g., sediment supply) on the FCN can be evaluated. We then do a preliminary test of the method using three fans in coastal British Columbia. The method uses a set of three measures: 1) the spatial extent of hydrogeomorphic activity, measured as the percentage of the contemporary fan surface area occupied by active channels; 2) the topologic structure, using node counts to measure channel network complexity; and 3) a sediment budget to indicate the proportion of sediment that is stored on the fan, loosely characterized by comparing the second-largest clast size of sediment at the fan apex or intersection point with the second- largest clast at the toe of the fan. These measures were applied to two fan deltas and one partial fan delta in northern Vancouver Island, British Columbia, using channel surveys conducted in 2006 and 2007. All three fans are located within 30 km of each other, have similar climatic, physiographic, and vegetation settings, and have natural channel networks. Each fan has perennial channel flow at the fan apex. Watershed areas range from 19.5 to 35.6 km 2 , and contemporary fan areas range from 0.2 to 0.7 km 2 . The Melton Relative Relief ratio ranges from 0.24 to 0.30 and none of the fans show evidence of debris floods or debris flows. In addition to testing the efficacy of the FCN measures, we use these fans to explore the question of whether fans in similar geomorphic settings and with similar controls develop similar FCNs. Results show the fans have between 4.7–8.5 % of the contemporary fan surface occupied by the active channel network. Topologic node counts indicate that the two fan deltas have a similar level of channel complexity, with 42–54 nodes in total. The partial fan delta channel network is approximately half as complex, with 23 nodes in total. All three fans have boulder-sized sediment (30–50 cm) at the fan apex. On the two fan deltas the largest sediment at the toe of the fan is 10–12 cm, indicating that much of the sediment delivered to the fan apex is stored on the fan. In contrast, the partial fan delta has 18–20 cm sized sediment transported into the axial valley river at the toe of the fan, and a wider and steeper outlet channel, indicating that a greater proportion of sediment is exported from the fan. Although the axial river does not visibly incise into the fan deposits, the lower channel complexity and coarser sediment output indicates that the axial river truncates the toe of the partial fan delta. The two fan deltas have similar FCNs, supporting the idea that fans in similar geomorphic settings develop similar FCNs. The partial fan delta is in a different geomorphic setting that was not identified prior to analysis of the FCNs, and has a different FCN. This study also examines the effects of a large regional storm event that occurred between the 2006 and 2007 surveys. One fan had increased active area, two fans had increased total node counts, and two fans increased output clast size after the flood event, indicating the measures are sensitive and respond to disturbance. Further investigations of FCNs may have utility in hazard assessments, addressing how fans respond to changes in sediment supply or discharge regime, how vegetation type affects FCNs, or the effect of other watershed processes or fan attributes. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. 1. Introduction Numerous investigators utilize the flexibility of physical or numeric models to study the controls of sediment, water discharge, vegetation, or other factors on fluvial fan development and the channel network (Gran and Paola, 2001; Cazanacli et al., 2002; Sun et al., 2002; Ashworth et al., 2004; Coulthard, 2005). Field-based studies with similar objectives are difficult to conduct given the difficulty in isolating individual controls, the often long time periods needed to observe changes, and the small number of fans that have natural channel networks unaltered by engineering works. Studies of Geomorphology 115 (2010) 286–293 ⁎ Corresponding author. E-mail address: Tom.Millard@gov.bc.ca (T.H. Millard). 0169-555X/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2009.06.029 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph