Hydrological functioning of a beaver dam sequence and regional dam persistence during an extreme rainstorm

Abstract

It is becoming increasingly popular to reintroduce beaver to streams with the hopes of restoring riparian ecosystem function or reducing some of the hydrological impacts of climate change. One of the risks of relying on beaver to enhance ecosystem water storage is that their dams are reportedly more apt to fail during floods which can exacerbate flood severity. Missing are observations of beaver dam persistence and water storage capacity during floods, information needed to evaluate the risk of relying on beaver as a nature‐based flood solution. A June rainstorm in 2013 triggered the largest recorded flood in the Canadian Rocky Mountains west of Calgary, Alberta. We opportunistically recorded hydrometric data during the rainfall event at a beaver‐occupied peatland that has been studied for more than a decade. We supplemented these observations with a post‐event regional analysis of beaver dam persistence. Results do not support two long‐held hypotheses—that beaver ponds have limited flood attenuation capacity and commonly fail during large flood events. Instead we found that 68% of the beaver dam cascade systems across the region were intact or partially intact after the event. Pond fullness, in addition to the magnitude of the water‐sediment surge, emerged as important factors in determining the structural fate of dam cascade sequences. Beaver ponds at the instrumented site quickly filled in the first few hours of the rain event and levels were dynamic during the event. Water storage offered by the beaver ponds, even ones that failed, delayed downstream floodwater transmission. Study findings have important implications for reintroducing beaver as part of nature‐based restoration and climate change adaptation strategies.

Beaver: Nature's ecosystem engineers

“…For example, see Nyssen et al (2011) who conducted one of the few in-channel hydrological studies of Eurasian beaver; finding that flow attenuation was in fact greatest during largest events. In 2013, Westbrook et al (2020) monitored the largest recorded flood in the Canadian Rocky Mountains west of Calgary, Alberta, challenging the commonly held assumption that dams fail during large floods (the majority fully or partially persisted) and showing that water storage offered by beaver dams (even failed ones) delayed downstream flood peaks. Therefore, it has been argued that the observed discontinuity or reduced downstream hydrological connectivity resulting from beaver dam-building activity-also shown by Butler and Malanson (2005), is a key reason for the flow attenuation impact persisting even for larger events during wetter periods (Puttock et al, 2017).…”

Beaver dam capacity of Canada’s boreal plain in response to environmental change

“…However, there is significant uncertainty around dam failure threshold. Some studies report that higher streamflow, especially during spring snowmelt, can lead to higher rates of beaver dam failure 64 while other studies report low rates of dam failure during large floods 65,66 . Thus, the dam failure threshold probably varies widely based on the state of repair of a dam, the materials from which the dam was built and pond fullness at the onset of a rain event 11 .…”

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

“…However, the limited range of test conditions makes these results highly preliminary. Interestingly, detailed field surveys from the Canadian Rockies found 31 of 74 dams (41%) could survive extreme flooding without impact, with failure rates amplified in more restricted river valleys (Westbrook et al, 2020). The large structural variation also highlights that beaver dams can spill (overtop) whilst retaining their integrity across a wide range of flow conditions, in which case they will revert to being important open channel roughness elements when submerged during floods, likely with considerable energy dissipation over the downstream side of the dam.…”

“…In all cases there was a rise in groundwater levels (as a result of increased ) following dam construction, and in the case of (Zahner, 1997) showed relatively rapid declines in level once the beaver dam was removed (Figure 9 a,b). In addition, depending on local topography and aquifer properties, recharge during flood events may be sufficient to cause local groundwater flooding, and thus contribute to the overall surface inundation (Westbrook et al, 2020). Interestingly, groundwater models have been under-utilized in examining potential impacts from beaver structures.…”

“…There is therefore a clear place for beavers in future landscape decisions concerning river corridors. Indeed, beavers have now entered, or are ready to enter, the lexicon of many restoration philosophies, most prominently: 'stage 0' (Cluer and Thorne, 2014), 'rewilding' (Law et al, 2017;Willby et al, 2018), 'nature based solutions' (Muller and Watling, 2016;Puttock et al, 2017;Westbrook et al, 2020), and 'ecosystem services' (Thompson et al, accepted), all of which are discussed in turn below. Although not synonymous, there is nonetheless considerable overlap between these concepts.…”