Not Just Another Dam Project By Lili-Maude Craig, Fisheries Technician
Beavers have been removed from much of their historical range in North America, and the consequences of their absence are apparent in arid landscapes like those found in southern Alberta. These ecosystems are at an increased risk for wildfires and drought, especially in today’s climate crisis. Therefore, the need for water retention provided by beaver dam complexes is significant. In the case of a tributary to Waiparous Creek (referred to as Whispering Pines West), the lack of beavers contributes to approximately 1.5km of the stream drying up every summer until mid-spring. This limits the habitat available to the at-risk Bull Trout (Salvelinus confluentus) and Westslope Cutthroat Trout (Oncorhynchus clarkii lewisi) found in this tributary.
Evidence of past beaver activity, such as abandoned lodges and old dams, is found where the original channel of Whispering Pines West used to flow. These structures are now covered in vegetation and appear as mere slopes on the land’s surface, indicating that beavers have not occupied this area in quite some time. The creek has since been realigned; we theorized that over time, it was captured by an adjoining off-highway vehicle trail, but it’s perhaps more likely that it was purposely diverted when a nearby gravel pit was built sometime in the 1970s. This straight and featureless reach has enabled water to travel at high speeds during high-flow periods, causing the banks to erode. The erosion has resulted in an over-widened, incised channel that becomes shallow and aggraded downstream.
Degraded reach of Whispering Pines West, unnamed tributary to Waiparous Creek
The creek’s entrenchment, or “downcutting”, lowered the water table and changed the composition of plant species growing within the riparian zone. The original channel was surrounded by a healthy willow stand which provided the stream with shade and the fish with cover from predators. The riparian vegetation now mainly consists of species that thrive in dry, well-drained soils (i.e., upland grasses and forbs), which provide neither of the services above. These species also have weak root systems, making the banks unstable and prone to failure. If left alone, the stream will continue to degrade over time, further disconnecting the floodplain and drying up the entire valley.
To mitigate these threats and improve fish habitat, Trout Unlimited Canada staff and volunteers installed Beaver Dam Analogs (BDAs) designed by Joal Borggard of Cascade Water Resources. BDAs are low-tech process-based restoration structures made entirely of natural materials that mimic the function of real beaver dams. They are designed to withstand annual floods but are not meant to be permanent structures. The BDAs will help address incision by promoting aggradation (filling in, elevating the stream channel) and floodplain reconnection. Water will be stored in the stream and adjacent soils for extended periods, which will help raise the water table and improve riparian health/function. Even a breached BDA will function with purpose, as the accumulation of sediment and debris will create habitat complexity.
Driving untreated wooden posts into the streambed using a gas-powered post-pounder
Dispersing cobble around the posts with a compact excavator
Construction of the BDAs began on September 6th, 2022 and took place over three days. The construction process included many steps that, when correctly applied, will improve the long-term performance of the structures. The first step was to drive two rows of untreated wooden posts into the stream bed, spanning the width of the channel. Cobble was dispersed around the posts, then willow and aspen branches were woven between. Aspen branches were also inserted through the weaves on the downstream end, parallel to the flow, to act as a cushion, preventing scour. The gap between the two rows of weave was filled with a 2:1 ratio of soil and mulch. The upstream end of the BDA was backfilled with a coarse substrate, followed by fine-grained material (silt), and then sod. The fill was wetted and compacted to reduce permeability and increase water-holding capacity. A total of 3 BDAs were built using this method to create a complex; this labour-intensive work surely increased appreciation for the free services provided by beavers!
Inserting aspen branches through the willow weave
Completed BDA (before posts were trimmed to height with a chainsaw)
Once the BDAs were complete, the aggraded reach slightly downstream needed to be restored. The upstream erosion caused this section to become too shallow from sediment deposition, dissipating flow. A new channel was dug out with the help of a compact excavator to reconnect the watercourse. Logs were cut to length and used to armor the new banks; they were anchored into place using untreated wooden posts. A gentle slope was created on the banks using shovels and pickaxes, exposing only one-third of the logs. Then, locally harvested live willow stakes were planted into the banks at a 45-degree angle to provide shade and cover when they grow. Additional features like cobble were also incorporated into the new channel. Water now has a defined path for continuous flow, and fish have more structure to shelter themselves from predators and swift currents.
Creating a slope on the bank of the log channel
Completed log channel immediately downstream of the BDAs
Habitat restoration is a priority when it comes to native trout recovery. The hard work put in by dedicated staff and volunteers during this project will recover hundreds of meters of habitat for federally threatened Bull Trout and Westslope Cutthroat Trout. Reconnecting this waterbody may also benefit other species, such as Mountain Whitefish (Prosopium williamsoni), Burbot (Lota lota) and Longnose Dace (Rhinichthys cataractae). Thanks to everyone’s contributions, the Beaver Dam Analogs, along with the removal of perched/undersized culverts further downstream, will provide these species with access to feeding grounds, spawning and overwintering habitat, and refugia during harsh environmental conditions. Hopefully, the landscape can recover enough to sustain the beavers’ recolonization so they can take over and save us from more arduous work in the future!
For more on this project, check out “This creek has a beaver problem” by Michael Short of “Let’s Go Outdoors”:
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