The beaver meadow complex revisited – the role of beavers in post‐glacial floodplain development

Abstract

We evaluate the validity of the beaver‐meadow complex hypothesis, used to explain the deposition of extensive fine sediment in broad, low‐gradient valleys. Previous work establishes that beaver damming forms wet meadows with multi‐thread channels and enhanced sediment storage, but the long‐term geomorphic effects of beaver are unclear. We focus on two low‐gradient broad valleys, Beaver Meadows and Moraine Park, in Rocky Mountain National Park (Colorado, USA). Both valleys experienced a dramatic decrease in beaver population in the past century and provide an ideal setting for determining whether contemporary geomorphic conditions and sedimentation are within the historical range of variability of valley bottom processes. We examine the geomorphic significance of beaver‐pond sediment by determining the rates and types of sedimentation since the middle Holocene and the role of beaver in driving floodplain evolution through increased channel complexity and fine sediment deposition. Sediment analyses from cores and cutbanks indicate that 33–50% of the alluvial sediment in Beaver Meadows is ponded and 28–40% was deposited in‐channel; in Moraine Park 32–41% is ponded sediment and 40–52% was deposited in‐channel. Radiocarbon ages spanning 4300 years indicate long‐term aggradation rates of ~0.05 cm yr‐1. The observed highly variable short‐term rates indicate temporal heterogeneity in aggradation, which in turn reflects spatial heterogeneity in processes at any point in time. Channel complexity increases directly downstream of beaver dams. The increased complexity forms a positive feedback for beaver‐induced sedimentation; the multi‐thread channel increases potential channel length for further damming, which increases the potential area occupied by beaver ponds and the volume of fine sediment trapped. Channel complexity decreased significantly as surveyed beaver population decreased. Beaver Meadows and Moraine Park represent settings where beaver substantially influence post‐glacial floodplain aggradation. These findings underscore the importance of understanding the historical range of variability of valley bottom processes, and implications for environmental restoration. Copyright © 2011 John Wiley & Sons, Ltd.

Mechanisms of carbon storage in mountainous headwater rivers

“…Radiocarbon ages on floodplain sediment are modern to depths of B20 cm, but dates on sediment from depths 430 cm reflect longer storage (ages ranging from 265±85 to 5,920±30 BP) (ref. 23). Large diameter dead wood requires more than a century to decay in this environment 24 .…”

Sediment and nutrient storage in a beaver engineered wetland

“…As long as supply continues, sediment will continue to accumulate until either the pond infills and sediments are colonised by plants forming a beaver meadow (Polvi and Wohl, 2012) or a dam collapses releasing sediment (Butler and Malanson, 2005). In catchments with high stream power, and associated risk of dam failure, there may be lower and less stable long‐term sediment associated stores of nutrients than presented herein (Błȩdzki et al ., 2011).…”

“…The most significant geomorphic impact of beavers results from their dam building ability and the consequent impoundment of large volumes of water and potentially associated sediment and nutrient accumulation in ponds (Naiman et al ., 1988; Butler and Malanson, 2005; Hood and Bayley, 2008). Dam and pond features can alter hydrological regimes, both locally and downstream (Polvi and Wohl, 2012; Burchsted and Daniels, 2014). The resulting increased structural heterogeneity of the environment (Rolauffs et al ., 2001) also creates a diverse range of habitats (Rosell et al ., 2005) with an increasingly recognised potential as a habitat restoration tool (Law et al ., 2017).…”

“…However, it must also be recognised that this beaver meadow end state is not reached in all situations and beaver dams can fail (Butler and Malanson, 2005). Typically during high energy rain events (Klimenko and Eponchintseva, 2015) beaver dam failure can result in releases of sediment (Polvi and Wohl, 2012, de Visscher et al ., 2014) meaning that sediment storage in ponds can be transient (Levine and Meyer, 2014). …”

Aerial photography collected with a multirotor drone reveals impact of Eurasian beaver reintroduction on ecosystem structure

“…Beavers are frequently described as ecological engineers (Hartman and Tornlov 2006), their greatest geomorphological impact being the construction of dams to impound water (Butler and Malanson 2005). Dam construction increases catchment hydrological storage capacity (Hammerson 1994;Hood and Bayley 2008), reduces stream velocity and peak discharge, altering flow regimes locally (Burchsted and Daniels 2014) and downstream (Polvi and Wohl 2012), so there is expected to be a positive impact on flood risk alleviation (Collen and Gibson 2000). Beavers also construct canals to facilitate safe access to foraging areas (Gurnell 1998), and the creation of wetlands and reduction in tree cover can increase biodiversity (see review: Rosell et al 2005).…”