Short and long-term changes to bed mobility and bed composition under altered sediment regimes Nira L. Salant a, * , Carl E. Renshaw a , Francis J. Magilligan b a Department of Earth Sciences, HB 6105, Dartmouth College, Hanover, NH, 03755, USA b Department of Geography, HB 6017, Dartmouth College, Hanover, NH, 03755, USA Received 13 May 2005; received in revised form 7 September 2005; accepted 15 September 2005 Available online 10 November 2005 Abstract Altered flow and sediment transport regimes from impoundment can result in significant channel bed composition changes that exacerbate the geomorphic and ecological effects of flow regulation. Using long-term discharge and cross-sectional data in combination with a two-fraction sediment transport model, we assess changes in the downstream bed of two flow-regulated rivers with equivalent dam-induced changes in flow but opposite changes in sediment flux. Supply limitation has led to incision and armoring in one case while supply excess has led to aggradation and embeddedness in the other. Under limited sediment supply, bed elevation variability decreases soon after impoundment, while excess sediment supply results in a decades-long gradual decrease in both bed elevation variability and depth of incision. Although the balance of sediment supply and transport differs between dam management styles, in both cases both the immediate and more gradual changes can be explained within the framework of a two-fraction sediment transport model. Our results demonstrate the importance of considering bed composition on sediment transport predictions and the development of management strategies for ecosystem maintenance. D 2005 Elsevier B.V. All rights reserved. Keywords: Flow-regulation; Channel morphology; Bed elevation; Sediment supply; Transport capacity 1. Introduction Attempts to mitigate the ecological impacts of dams from large-scale changes in the hydrologic and sedi- ment regime (Collier, 2002; Osmundson et al., 2002) have focused on releasing flushing flows capable of loosening the gravel bed and mobilizing fine sediment (Kondolf and Wilcock, 1996). Because of the high costs of flushing flows and the sensitivity of transport thresholds, accurately defining the duration, frequency, and magnitude of these flows is crucial (Topping et al., 2000a). Pre-dam hydrologic records can contextualize the frequency and magnitude of discharges required to maintain natural variability in channel bed morphology as they provide a normative index of pre-impact equi- librium between sediment supply and sediment trans- port. However, the strong coupling between sediment flux and fluvial characteristics means that post-dam geomorphic adjustments, particularly in overall bed composition, may alter the bnaturalQ (i.e. pre-dam) re- lationship between discharge and channel maintenance (Dietrich et al., 1989; Lisle et al., 1993, 2000; Topping 0169-555X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2005.09.003 * Corresponding author. Tel.: +1 604 619 1623; fax: +1 604 822 6150. E-mail addresses: nira@alum.dartmouth.org (N.L. Salant), carl.renshaw@dartmouth.edu (C.E. Renshaw), fjm@dartmouth.edu (F.J. Magilligan). Geomorphology 76 (2006) 43 – 53 www.elsevier.com/locate/geomorph