BEAVER ASSISTED RIVERVALLEY FORMATION C. J. WESTBROOK, a * D. J. COOPER b and B. W. BAKER c a Centre for Hydrology, Department of Geography & Planning, University of Saskatchewan, Saskatoon, Saskatchewan, Canada b Graduate Degree Program in Ecology, Colorado State University, Fort Collins, Colorado, USA c US Geological Survey, Fort Collins Science Center, Fort Collins, Colorado, USA ABSTRACT We examined how beaver dams affect key ecosystem processes, including pattern and process of sediment deposition, the composition and spatial pattern of vegetation, and nutrient loading and processing. We provide new evidence for the formation of heterogeneous beaver meadows on riverine system floodplains and terraces where dynamic flows are capable of breaching in-channel beaver dams. Our data show a 1.7-m high beaver dam triggered overbank flooding that drowned vegetation in areas deeply flooded, deposited nutrient-rich sediment in a spatially heterogeneous pattern on the floodplain and terrace, and scoured soils in other areas. The site quickly de-watered following the dam breach by high stream flows, protecting the deposited sediment from future re-mobilization by overbank floods. Bare sediment either exposed by scouring or deposited by the beaver flood was quickly colonized by a spatially heterogeneous plant community, forming a beaver meadow. Many willow and some aspen seedlings established in the more heavily disturbed areas, suggesting the site may succeed to a willow carr plant community suitable for future beaver re-occupation. We expand existing theory beyond the beaver pond to include terraces within valleys. This more fully explains how beavers can help drive the formation of alluvial valleys and their complex vegetation patterns as was first postulated by Ruedemann and Schoonmaker in 1938. Copyright # 2010 John Wiley & Sons, Ltd. key words: beaver meadow; Castor canadensis; ecosystem engineering; floodplain; habitat heterogeneity; river valley formation; Rocky Mountain National Park; terrace Received 14 July 2009; Revised 23 October 2009; Accepted 9 December 2009 INTRODUCTION Beaver (Castor canadensis and C. fiber) are key ecological agents that shape riverine landscapes by influencing the hydrogeomorphic processes that connect floodplain ecosys- tems to streams (Gurnell, 1998). Beaver build dams across low order streams to pond water and provide protection from predators, expand foraging areas and territories, and store food over winter months (Baker and Hill, 2003). Dam building results in the transformation of terrestrial to wetland and lotic to lentic ecosystems, which alter patterns of sediment retention (Naiman et al., 1986; Butler and Malanson, 2005), rates of soil nutrient cycling (Naiman et al., 1994; Johnston et al., 1995), organic matter and nutrient deposition and retention (Naiman and Melillo, 1984; Devito and Dillon, 1993) and vegetation succession (Terwilliger and Pastor, 1999). Historically beaver were abundant and widely distributed in North America and Eurasia. Although their populations were significantly decreased by fur trapping, they have recovered in much of North America and are increasing in Europe. Beaver have been shown to influence up to 30–50% of the total length of 5th order and smaller streams (Naiman and Melillo, 1984) in some locations. It is therefore critical to understand the influence of beaver on hydrogeomorphic processes, particu- larly on headwater streams which, for example, comprise 85% of total stream length in the USA (Leopold et al., 1964). The existing model of beaver meadow formation provides a conceptual framework for testing the hydrogeomorphic influence of beaver on riverine landscapes. Beaver alter plant communities by building dams that impound water in upstream ponds and reduce stream velocity. Ponds can also inundate adjacent riparian areas, which may drown existing vegetation. They also function as efficient water and sediment traps, and may fill with fine-grained, nutrient-rich sediment and organic materials (Bigler et al., 2001). Sediment can remain in the pond area when dams are abandoned on small streams (Butler and Malanson, 2005) or if there is non-repairable channel evulsion around the dam while it is active (Cooper et al., 2006). The water table recedes as dams degrade and the bare, moist, nutrient-rich sediment of abandoned ponds is quickly colonized by herbaceous plants or shrubs, forming what has been termed a beaver meadow (Ives, 1942; Johnston and Naiman, 1987). Beaver meadows are relatively homogeneous patches of distinct plant communities that increase heterogeneity at the landscape scale (Remillard et al., 1987; McMaster and McMaster, 2000; Wright et al., 2002). Beaver meadows may RIVER RESEARCH AND APPLICATIONS River. Res. Applic. (2010) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rra.1359 *Correspondence to: C. J. Westbrook, Centre for Hydrology, Department of Geography & Planning, University of Saskatchewan, 117 Science Place, Saskatoon, SK S7N 5C8, Canada. E-mail: cherie.westbrook@usask.ca Copyright # 2010 John Wiley & Sons, Ltd.