Beaver‐mediated water table dynamics in a Rocky Mountain fen

Abstract

Beaver dams are known to raise water tables in mineral soil environments, but very little is known about their impact in wetlands, such as peatlands. Peatlands tend to have shallow water tables, and the position and tendency of the water table to fluctuate (i.e., stability) is a factor controlling the system's ability to store carbon and water. Many peatland environments, especially fens, offer ideal habitat for beaver, and the potential for beaver dams to influence this link by manipulating water tables requires investigation. Our objective was to determine the influence of beaver dams on the water table dynamics of a Rocky Mountain fen. We monitored water tables in the peatland for 4 years while beaver dams were intact and 2 years after they were breached by an extreme flood event. We found that, because of the unique way in which dams were built, they connected the peatland to the stream and raised and stabilized already high water tables within a 150‐m radius. Beaver‐mediated changes to peatland water table regimes have the potential to enhance carbon sequestration and the peatland's ability to respond to external pressures such as climate change. Furthermore, beaver dams increased surface and groundwater storage, which has implications for regional water balances, especially in times of drought.

Hydrological Function of a Mountain Fen at Low Elevation Under Dry Conditions

“…At the end of data set, water tables at W4 and W7 were 0.45 and 0.58 m, respectively, lower than water tables at W61 and W62. Of note, wells 61 and 62 were within the 150‐m zone of influence of a beaver dam; water tables are elevated 13 cm on average when in this zone of influence (Karran et al, ). From 28 June to the beginning of August, well 4 and 7 water tables declined at an average rate of 13 mm/day, and well 62 and 62 water tables declined at a rate of 5 mm/day.…”

“…Water from the beaver ponds may have bypassed the dams by way of throughflow or escaping through weakened areas of dam construction (Westbrook, Cooper, & Baker, ; Woo & Waddington, ). Sibbald fen usually has many beaver ponds (Karran et al, ; Morrison et al, ), and the few present during the study likely influenced the observed gradual return of streamflow after rainstorms.…”

“…As Sibbald fen has been an active research site since 2006 (Westbrook & Bedard‐Haughn, ), several long‐term installations have been put in place to gather hydrometric data (Figure ). Specifically, there is a large well network, consisting of 51 wells along 11 transects, available for measuring water table (Karran et al, ). Wells were constructed of 4 cm diameter, fully slotted PVC pipe, capped at the bottom and installed with a hand auger to between 0.6 to 1.8 m depth, depending on the thickness of the peat and the range in water table position.…”

“…The horizontal groundwater hydraulic gradient at Sibbald fen is oriented in the down‐valley (north to south) direction (Karran et al, ). Lateral groundwater flows, G l , were determined using hydraulic information for pairs of wells in the north and south ends of the fen in Darcy's Law: …”

“…Fens though can strongly regulate runoff when thawed (Quinton & Carey, ; Wells, Ketcheson, & Price, ), particularly when the water table is low (Goodbrand et al, ; Spence, Guan, & Phillips, ). Water tables of mountain fens are highly dynamic (Karran, Westbrook, & Bedard‐Haughn, ; Millar, Cooper, & Ronayne, ). Some fens have water tables that remain near the surface throughout the summer while others experience strong water table declines.…”