Return flows from beaver ponds enhance floodplain-to-river metals exchange in alluvial mountain catchments

Abstract

River to floodplain hydrologic connectivity is strongly enhanced by beaver-(Castor canadensis) engineered channel water diversions. The hydroecological impacts are wide ranging and generally positive, however, the hydrogeochemical characteristics of beaver-induced flowpaths have not been thoroughly examined. Using a suite of complementary ground-and drone-based heat tracing and remote sensing methodology we characterized the physical template of beaver-induced floodplain exchange for two alluvial mountain streams near Crested Butte, Colorado, USA. A flowpath-oriented perspective to water quality sampling allowed characterization of the chemical evolution of channel water diverted through floodplain beaver ponds and ultimately back to the channel in 'beaver pond return flows'. Return seepages were universally suboxic, while ponds and surface return flows showed a range of oxygen concentration due to in-situ photosynthesis and atmospheric mixing. Median concentrations of reduced metals: manganese (Mn), iron (Fe), aluminum (Al), and arsenic (As) were substantially higher along beaver-induced flowpaths than in geologically controlled seepages and upstream main channel locations. The areal footprint of reduced return flow seepage flowpaths were imaged with surface electromagnetic methods, indicating extensive zones of high-conductivity 2 2 shallow groundwater flowing back toward the main channels and emerging at relatively warm bank seepage zones observed with infrared. Multipledepth redox dynamics within one focused seepage zones showed coupled variation over time, likely driven by observed changes in seepage rate that may be driven by pond stage. High-resolution times series of dissolved Mn and Fe collected downstream of the beaver-impacted reaches indicated seasonal dynamics in mixed river metal concentrations. Al time series concentrations showed proportional change to Fe at the smaller stream location, indicating chemically reduced flowpaths were sourcing Al to the channel. Overall our results indicated beaver-induced floodplain exchanges create important, and perhaps dominant, transport pathways for floodplain metals by expanding chemically-reduced zones paired with strong advective exchange.

Characterizing Physical Properties of Streambed Interface Sediments Using In Situ Complex Electrical Conductivity Measurements

“…Our selected field study highlights one specific application of CC, being a rapid, noninvasive technology for identifying anoxic groundwater discharge zones resulting in high concentrations of Fe(III) oxides, especially when Fe(III) oxides are not visible due to deep or turbid water, aquatic vegetation, or shallow burial. These discharge zones can occur frequently when the groundwater is impacted by anthropogenic contamination such as landfill leachate (Bakyayita et al., 2019) and mining activities (Gandy et al., 2007) or natural processes such as beaver activity (Briggs et al., 2019) and organic‐rich peat deposits (Briggs et al., 2018b). Those processes consume the groundwater dissolved oxygen and increase the dissolved ions either by direct inputs or reducing the aquifer geological minerals, which result in unique discharge zones considered as hotspots for biogeochemical reactions (Boano et al., 2014; Vidon et al., 2010).…”

“…In an additional application of CC measurements to a zone of known anoxic floodplain seepage along the larger alluvial East River (Colorado, USA), we found a similar correlation between and evidence of Fe(III) oxide precipitates to that observed in the Mashpee River. In East River, anoxic groundwater discharges were induced by floodplain beaver pond subsurface return flows, which produced visible rust‐stained sediments indicating Fe(III) oxide precipitates (Briggs et al., 2019) (Figure ). Distinctly higher (at 4 Hz) was acquired over these rust‐stained sediments (2.3 ± 0.77 μS cm −1 from 22 different sites) relative to the more typically alluvial sediments along the main river channel (0.26 ± 0.37 μS cm −1 from 14 different sites) (Figure ).…”

“…sed , Briggs et al (2019) documented highly reduced floodplain groundwater upwelling with low dissolved oxygen, low redox potential and elevated concentrations of dissolved iron, manganese, aluminum, and arsenic. The East River CC results support the transferability of CC measurements for detecting zones with high concentrations of Fe(III) oxides that may represent geochemical hotspots throughout different river corridor settings.…”

“…Streambed precipitation of Fe(III) oxides is observed downstream of discharges of leachate from both mine tailings (Gandy et al., 2007) and landfills (Bakyayita et al., 2019). Fe(III) oxides are also precipitated at anoxic discharge zones resulting from natural processes, including those associated with floodplain beaver ponds (Briggs et al., 2019). Rapid, noninvasive characterization of Fe(III) oxide distributions would aid identification of anoxic groundwater discharge zones associated with such processes and could enhance understanding of contaminant transport through river corridors where high surface area Fe(III) oxides function as sorption sinks for a range of stream water contaminants (e.g., arsenic, uranium) (Fuller & Harvey, 2000; Gandy et al., 2007).…”

Dam busy: beavers and their influence on the structure and function of river corridor hydrology, geomorphology, biogeochemistry and ecosystems

“…The cumulative effects of these expanded pathways are not well known in beaver systems, but it is nonetheless a mechanism to increase the concentration of Fe 3+ and associated metals and nutrients in solution, which may then in turn be re-oxidised by a variety of abiotic and biological mechanisms if these pathways re-enter downstream anaerobic surface waters (Figure 15). Indeed, Briggs et al (2019) found that beaver dam induced diversion of water across a floodplain resulted in subsequent return flow to the main channel that was variable in redox status and substantially enriched in iron, manganese, aluminium, and arsenic concentrations. Some combination of expanded anaerobic conditions and flow mixing may thus lead to the enhancement of Fe 2+ concentrations downstream of beaver systems, which Cirmo and Driscoll (1993) found could be up to four times higher than inflowing concentrations.…”