Beaver dams and channel sediment dynamics on Odell Creek, Centennial Valley, Montana, USA Rebekah Levine ⁎, Grant A. Meyer Department of Earth and Planetary Sciences, 1 University of New Mexico, MSCO3-2040, Albuquerque, NM 87131, United States abstract article info Article history: Received 12 July 2011 Received in revised form 14 April 2013 Accepted 17 April 2013 Available online 7 May 2013 Keywords: Channel morphology Sediment storage Beaver dams Aggradation Dam breach Beaver dams in streams are generally considered to increase bed elevation through in-channel sediment stor- age, thus, reintroductions of beaver are increasingly employed as a restoration tool to repair incised stream channels. Here we consider hydrologic and geomorphic characteristics of the study stream in relation to in-channel sediment storage promoted by beaver dams. We also document the persistence of sediment in the channel following breaching of dams. Nine reaches, containing 46 cross-sections, were investigated on Odell Creek at Red Rock Lakes National Wildlife Refuge, Centennial Valley, Montana. Odell Creek has a snowmelt-dominated hydrograph and peak flows between 2 and 10 m 3 s -1 . Odell Creek flows down a flu- vial fan with a decreasing gradient (0.018–0.004), but is confined between terraces along most of its length, and displays a mostly single-thread, variably sinuous channel. The study reaches represent the overall down- stream decrease in gradient and sediment size, and include three stages of beaver damming: (1) active; (2) built and breached in the last decade; and (3) undammed. In-channel sediment characteristics and stor- age were investigated using pebble counts, fine-sediment depth measurements, sediment mapping and sur- veys of dam breaches. Upstream of dams, deposition of fine (≤2 mm) sediment is promoted by reduced water surface slope, shear stress and velocity, with volumes ranging from 48 to 182 m 3 . High flows, however, can readily transport suspended sediment over active dams. Variations in bed-sediment texture and channel morphology associated with active dams create substantial discontinuities in downstream trends and add to overall channel heterogeneity. Observations of abandoned dam sites and dam breaches revealed that most sediment stored above beaver dams is quickly evacuated following a breach. Nonetheless, dam remnants trap some sediment, promote meandering and facilitate floodplain development. Persistence of beaver dam sediment within the main channel on Odell Creek is limited by frequent breaching (b 1–5 years), so in-channel sediment storage because of damming has not caused measurable channel aggradation over the study period. Enhanced overbank flow by dams, however, likely increases fine-grained floodplain sedimen- tation and riparian habitat. Contrasts between beaver-damming impacts on Odell Creek and other stream systems of different scales suggest a high sensitivity to hydrologic, geomorphic, and environmental controls, complicating predictions of the longer-term effects of beaver restoration. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Fluvial and riparian habitats are hubs of biodiversity and essential habitat at the land–water interface in the semi-arid western United States. Riverine and associated habitats are subject to disturbance by changing river flows (Beever et al., 2005) and because of relatively small area but high ecological significance, are areas of primary con- cern for land managers. Thus, the interaction between physical and biological components of river systems is an active area of research (e.g., Petts, 2009). Beaver damming is thought to be an effective mechanism for reconnecting incised streams to historic floodplains because of the propensity for sediment to be trapped upstream of dams in the beaver ponds (Beechie et al., 2008). Research on the in-channel dynamics of beaver dams and the effects on sediment transport, however, is limited, and few studies have attempted to quantify the persistence of sediment within the channel, the location of maximum storage, and the caliber of the sediment stored. Historical accounts indicate that North American beaver (Castor canadensis) dams had much greater importance in fluvial systems prior to European colonization and extensive beaver trapping (Pollock et al., 2003; Wohl, 2006). Pre-colonization beaver popula- tions are estimated at between 60 and 400 million (Seton, 1929; Naiman et al., 1988), compared with estimates today of 6–12 million (Naiman et al., 1988). Beaver damming has been shown to increase riparian vegetation, raise water levels, attenuate flood peaks and alter sediment transport and storage patterns (e.g., McCullough et al., 2005). Thus, the boggy, flooded landscapes and extensive riparian zones associated with beaver damming are likely reduced at present and represent one of the major human alterations to fluvial landscapes. Geomorphology 205 (2014) 51–64 ⁎ Corresponding author. Tel.: +1 505 277 4204; fax: +1 505 277 8843. E-mail addresses: rebekahl@unm.edu (R. Levine), gmeyer@unm.edu (G.A. Meyer). 0169-555X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geomorph.2013.04.035 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph