This is a pre-print and is undergoing peer-review. Manuscript submitted for publication in Earth Science Reviews 1 Dam busy: beavers and their influence on the structure and function of river 2 corridor hydrology, geomorphology, biogeochemistry and ecosystems 3 4 5 Annegret Larsen1,2*, Joshua Larsen3,4*, Stuart N. Lane5 6 1 Soil Geography and Landscape group, Wageningen University, Droevendaalsesteeg 3, 7 6708 PB Wageningen, The Netherlands, [email protected] (corresponding author) 8 2 The University of Manchester, Oxford Rd, Manchester, M13 9PL, United Kingdom 9 3 School of Geography, Earth and Environmental Sciences, University of Birmingham, 10 United Kingdom 11 4 Birmingham Institute of Forestry Research (BIFoR), University of Birmingham, United 12 Kingdom 13 5 Institute of Earth Surface Dynamics, Université de Lausanne, 1015 Lausanne, Switzerland 14 *these authors have contributed equally to this work 15 16 This manuscript has been submitted for publication in the journal Earth Science Reviews. 17 Please note that the manuscript is currently under peer-review, and has yet to be accepted 18 for publication. Subsequent versions of this manuscript may have slightly different 19 content. If accepted, the final version of the manuscript will be available via the peer- 20 review DOI link on the right-hand side of the webpage. Please feel free to contact any of 21 the authors, we welcome feedback. 22 23 24 Keywords 25 Beaver, hydrology, geomorphology, biogeochemistry, water quality, carbon, ecosystem, disturbance, 26 river restoration, ecosystem engineering, keystone species 27 28 1 Abstract 29 Beavers (castor fiber, castor canadensis) are the most influential mammalian ecosystem engineer, 30 heavily modifying river corridors and influencing hydrology, geomorphology, nutrient cycling, and 31 ecosystems. As an agent of disturbance, they achieve this first and foremost through dam construction, 32 which impounds flow and increases the extent of open water, and from which all other landscape and 33 ecosystem impacts follow. After a long period of local and regional eradication, beaver populations 34 have been recovering and expanding throughout Europe and North America, as well as an introduced 35 species in South America, prompting a need to comprehensively review the current state of knowledge 1 This is a pre-print and is undergoing peer-review. Manuscript submitted for publication in Earth Science Reviews 36 on how beavers influence the structure and function of river corridors. Here, we synthesize the overall 37 impacts on hydrology, geomorphology, biogeochemistry, and aquatic and terrestrial ecosystems. Our 38 key findings are that a complex of beaver dams can increase surface and subsurface water storage, 39 modify the reach scale partitioning of water budgets, allow site specific flood attenuation, alter low 40 flow hydrology, increase evaporation, increase water and nutrient residence times, increase 41 geomorphic heterogeneity, delay sediment transport, increase carbon, nutrient and sediment storage, 42 expand the extent of anaerobic conditions and interfaces, increase the downstream export of 43 dissolved organic carbon and ammonium, decrease the downstream export of nitrate, increase lotic 44 to lentic habitat transitions and aquatic primary production, induce ‘reverse’ succession in riparian 45 vegetation assemblages, and increase habitat complexity and biodiversity on reach scales. We then 46 examine the key feedbacks and overlaps between these changes caused by beavers, where the 47 decrease in longitudinal hydrologic connectivity create ponds and wetlands, transitions between lentic 48 to lotic ecosystems, increase vertical hydraulic exchange gradients, and biogeochemical cycling per 49 unit stream length, while increased lateral connectivity will determine the extent of open water area 50 and wetland and littoral zone habitats, and induce changed in aquatic and terrestrial ecosystem 51 assemblages. However, the extent of these impacts depends firstly on the hydro-geomorphic 52 landscape context, which determines the extent of floodplain inundation, a key driver of subsequent 53 changes to hydrologic, geomorphic, biogeochemical, and ecosystem dynamics. Secondly, it depends 54 on the length of time beavers can sustain disturbance at a given site, which is constrained by top down 55 (e.g. predation) and bottom up (e.g. competition) feedbacks, and ultimately determines the pathways 56 of river corridor landscape and ecosystem succession following beaver abandonment. This outsized 57 influence of beavers on river corridor processes and feedbacks is also fundamentally distinct from what 58 occurs in their absence. Current river management and restoration practices are therefore open to re- 59 examination in order to account for the impacts of beavers, both positive and negative, such that they 60 can potentially accommodate and enhance the ecosystem engineering services they provide. It is 61 hoped that our synthesis and holistic framework for evaluating beaver impacts can be used in this 62 endeavor by river scientists and managers into the future as beaver populations continue to expand in 63 both numbers and range. 64 65 Contents 66 1 Abstract ......................................................................................................................................1 67 2 Introduction ................................................................................................................................4 68 3 Beaver impacts on hydrology ......................................................................................................6 69 3.1 Changes to storage and open water area .............................................................................7 70 3.2 Water balance .....................................................................................................................9 71 3.3 High flow and flood impacts .............................................................................................. 11 72 3.4 Low flow impacts ............................................................................................................... 14 73 3.5 Ground and surface water interactions .............................................................................. 15 74 3.6 Hyporheic exchange .......................................................................................................... 17 75 3.7 Water residence times ....................................................................................................... 19 2 This is a pre-print and is undergoing peer-review. Manuscript submitted for publication in Earth Science Reviews 76 3.8 Water temperature ........................................................................................................... 20 77 4 The influence of beavers on river-floodplain Geomorphology ................................................... 21 78 4.1 Sediment transport and deposition in beaver systems ....................................................... 22 79 4.2 Erosion in beaver systems.................................................................................................. 25 80 4.3 The role of beaver channels, burrows and dams in the hydro-geomorphology of rivers and 81 floodplains .................................................................................................................................... 26 82 4.4 Decadal to millennial valley formation mediated by beavers.............................................. 28 83 5 Changes in biogeochemistry, carbon and nutrient cycling, and water quality ............................ 30 84 5.1 Changes to biogeochemical pathways ............................................................................... 30 85 5.2 Beaver impacts on the carbon cycle ................................................................................... 31 86 5.3 Beaver impacts on the Nitrogen cycle ................................................................................ 36 87 5.4 Beaver impacts on the Phosphorus cycle ........................................................................... 38 88 5.5 Impacts on iron cycling, mercury, and additional contaminants ......................................... 39 89 5.6 Impacts on source vs sink behavior, and the evolution of overall water quality and its 90 variability ...................................................................................................................................... 40 91 6 Beaver impacts on aquatic and riparian ecosystems .................................................................. 42 92 6.1 Creating a mix of lotic and lentic environments, disruptions to the river continuum, and 93 changes to aquatic ecosystem productivity ................................................................................... 42 94 6.2 Beaver impacts on ecosystem biodiversity and functioning: Macro-Invertebrates ............. 44 95 6.3 Beaver impacts on ecosystem biodiversity and functioning: Fish........................................ 45 96 6.4 Beaver impacts on ecosystem biodiversity and functioning: Other fauna ........................... 48 97 6.5 Beaver impacts on ecosystem biodiversity and functioning: Vegetation ............................. 48 98 7 Interconnections and feedbacks between the hydrology, geomorphology, biogeochemistry and 99 ecosystems of beaver impacted streams ........................................................................................... 52 100 7.1 Initial