A Beaver Dam Analog (BDA) is a man-made structure designed to mimic the form and function of a natural beaver dam. In general the goal of BDAs is to create the conditions that that promote physically complex streams that lead to ecological health. BDAs can also be used to increase the probability of successful beaver translocation by creating immediate deep water habitat that reduces the risk of predation which can decrease their desire to move from the desired restoration location. In general, the design and installation of BDA complexes is a simple, cost-effective, non-destructive approach to stream restoration that can influence a suite of hydraulic and hydrologic processes in order to achieve a range of common restoration goals.
A BDA complex (hereafter: complex) is typically composed of 2-8 individual BDAs that are designed to work together in order to achieve restoration at the reach scale. Restoration goals such as: improved habitat complexity, sediment retention, riparian quality and extent, elevated baseflows, increased fish survival and population growth are reach and watershed scale goals. Individual BDAs, while influential locally, must be pervasive enough (i.e., high density) and work together in order to achieve these goals. Therefore, while the following section contains information on the objectives, function and design considerations of individual structures, it is important to recognize the restoration goals are articulated at the complex and multi-complex scale, and it is these goals that determine the individual structure built. Insert 2 photos: 1 of beaver dam, 1 of BDA and annotate both to show effects
While many BDAs utilize 3” untreated are constructed of untreated, sharpened wooden fence posts, native woody vegetation, and a limited amount of fill material (typically native sediment from the surrounding area) (Figure 3 and 4). Wooden posts approximately 3-4” diameter are driven into the streambed and banks using a hydraulic post pounder (Figure 4). Posts extend roughly 1 m above the channel bed, are driven up to 1 m into the streambed and are spaced approximately 0.3 - 0.5 m apart. Locally available woody vegetation is woven between the posts to create a semipermeable structure and sediment (sand, gravel, and cobble) from adjacent hillslopes and floodplains is placed at the base to protect the posts from scour and decrease the permeability of the structure, promoting upstream pond formation. The life-span of a BDA is determined by local factors (e.g. sediment load, channel geometry etc.), but is generally < 5 years, or the time it takes for the dam to fill with sediment and be colonized by riparian vegetation. BDA are clustered into complexes that are generally comprised of 2-8 individual structures. Building complexes mimics natural beaver dam activities and promotes spatially extensive, non-local influence on hydrologic and geomorphic processes. The spacing between structures is similar to the dam layout of a natural beaver colony (approximately 30 - 100 m between structures), and depends on stream gradient, width and restoration objectives.
The construction of BDAs does not require detailed drawings or high resolution precision. Because BDAs are intended to mimic beaver dams, and are not intended to be permanent structures the design and construction does not require the type of assessment generally demanded of more highly engineered, permanent structures. The construction specifications of any individual structure are driven by the intended function of the structure (i.e., processes to be influenced) and the structure location. Because BDA treatments are intended to exist over longer stream lengths and work together, individual structure placement and restoration objectives both influence each other, locations for structures are selected based on the likelihood that such a location would promote specific outcomes. Furthermore, while individual structures may exert significant local influence, BDA complexes, and restoration projects are specifically designed to be dynamic, rather than static features, and the “failure” of structure may often times provide many of the same benefits as an intact structure (e.g., increased hydraulic diversity, geomorphic complexity, and roughness that promotes floodplain connectivity). The design and construction of individual BDA structures can be accomplished in the field without the help of advanced software in only a few hours.
Because of the low capital costs of BDAs restoration projects can treat many more kilometers of stream, which increases the probability of achieving larger scale restoration objectives, especially goals related to water storage and ecosystem health which necessarily require larger treatment areas.
BDAs mimic natural beaver dams, as such they require minimal disturbance of the channel, and no heavy machinery.
BDAs are one tool that can be used in cheap and cheerful restoration projects to facilitate the reintroduction of beaver, support existing beaver populations or attempt to capture some of the benefits of natural beaver dams in the absence of beaver. BDAs can be used to 1) create the conditions required for translocating beaver by influence stream hydrology to improve riparian areas 2) accelerate the recovery of incised channels by channel widening or aggradation 3) Force ponding, providing habitat and protection from predators for immediate beaver reintroduction 4) Increase the probability that beaver will not emigrate from the restoration/translocation site by providing immediate habitat and refuge 5) influence a suite of hydrogeomorphic processes that are essential to dynamic and healthy stream ecosystems, including
* Increased baseflow * Elevated water tables * Increased channel-floodplain connectivity * Improved riparian extent and condition * Channel incision recovery
There are multiple types of BDAs, each designed to preferentially influence specific processes, and each resembling natural beaver dams. Each type of dam has specific design considerations that are determined by restoration objectives.
BDA type Principle Objectives & Function Design Construction Primary Dam Force extensive upstream ponding, provide water storage, encourage aggradation, force overbank flows during high flow events, increase groundwater recharge Channel spanning dam that extends onto adjacent floodplain, crest elevation high enough to force flow onto inset floodplain and benches. Convex post-line with woody vegetation weave, sediment placed at upstream base of dam to promote pond formation and increase stability, downstream mattress for scour prevention Secondary Dam Increase stability of primary dam by reducing hydraulic gradient, capture return flow, increase areal extent of ponding, promote aggradation Channel spanning dam installed generally installed downstream of primary dam, lower crest elevation than primary dam Straight post line with woody vegetation weave, less extensive sediment buttressing, little to no downstream mattress Constriction Dam Recruit sediment via bank erosion and/or scour pool formation, increase width of incision trench Spans 50-90% of the channel to force flow constriction Straight post line with woody vegetation weave and sediment at upstream base, can be bank attached or mid-channel Reinforced Existing Dam Increase stability of an existing beaver dam to increase lifespan of dam and beaver survival Active or abandoned dams that can promote beaver (re)colonization or influence processes that promote restoration objectives Post-line equal to width of dam, posts are installed within existing structure just downstream of dam crest
Primary dams are the largest BDA structures, extend across the entire channel and sometimes onto adjacent surfaces that can be inundated (e.g. inset floodplain, benches and floodplain).
The objective of primary dams is to cause immediate ponding upstream in order to:
In locations where natural beaver dams area absent or have a short residence time primary dams provide immediate ponding for beaver colonization. They reduce the upstream water surface gradient and promote flow dispersal onto adjacent surfaces, reducing the likelihood of dam breaching during high flow events. Ponding also provides fish cover and flow and thermal refugia. In incised streams, channel aggradation upstream of BDAs promotes channel-floodplain connectivity by increasing channel bed elevation. Primary dams increase geomorphic complexity by causing upstream pond formation and aggradation and downstream scour and bar formation. Increases in geomorphic complexity, overbank flows, and groundwater elevation may also promote the recruitment and growth of riparian plant species which promotes a positive feedback for fish habitat by increasing cover and large wood inputs to the river.
Site selection and design for primary dams is driven by the restoration objectives. The major considerations for installation include structure placement within the treatment reach and dam crest elevation.
Structure placement is primarily determined by channel morphology, including:
Dam crest elevation (dam height) is determined by a combination of local channel morphology and restoration objectives, including:
All BDA structures utilize a line of 3-4” sharpened, untreated wooden posts driven into the streambed through which willow and other locally available woody material can be woven. Primary dams differ from other BDA structures is several important ways: • Convex post line – convex post line orientation dissipates flow over the dam crest and to prevent excessive scour downstream of BDA • Impermeable base – locally available sediment, ideally a combination of fine and coarse grained, is used to protect the base of the structure as well as cause immediate upstream ponding • Mattress construction – woody vegetation, placed parallel to flow, and sediment is placed on the downstream side of the structure in order to protect against excessive scour and/or head-cutting
Secondary dams are smaller than primary dams, both vertically and laterally, and are generally built downstream of primary dams. Secondary dams may serve some of the same functions as primary dams, but their principal purpose is to increase the stability of the primary dam.
The principal objectives of secondary dams are to:
Objectives shared by secondary dams and primary dams include:
In general the location and design of secondary dams takes into consideration many of the same morphological and channel geometry concerns as primary dams, including channel width, depth and presence/absence of surfaces that can be inundated. Secondary dams are not as wide as primary dams, typically do not extend onto the floodplain, and the crest elevation is lower than that of primary dams. Design considerations specific to secondary dams are driven by the purpose of the individual structure:
Construction of secondary dams is very similar to that of primary dams and requires the same methods and materials outlined in the previous section. However, because secondary dams generally have a lower crest elevation and support less extensive ponds several of the measures used to ensure the stability of primary dams can be relaxed when constructing secondary dams.
Unlike primary and secondary dams, constriction dams are not designed to impound water, and therefore do not extend across the entire channel (Figure 7). Constriction dams mimic failed beaver dams and therefore play an important role in the evolutionary cycle of incised streams (Pollock et al., 2014). Constriction dams force a constriction in the river creating a hydraulic jet. A hydraulic jet has a greater capacity to do geomorphic work and can be strategically directed towards erodible banks or existing structural elements in order to meet restoration objectives. Constriction dams increase instream geomorphic complexity both directly and indirectly in a number of ways:
Constriction dams generally span 50-90% of the channel and are oriented downstream at ~120° however they may also be located mid-channel where they create hydraulic jets on both sides of the structure. In such cases they have also been called Post-Assisted Log Structures (PALS). A secondary dam with end-cuts on both sides is effectively a constriction dam. Site selection for constriction dams should consider:
Construction of constriction dams relies on similar methods used to construct both primary and secondary dams, with a few important differences:
While constriction dams are built with the explicit objective of sediment recruitment and/or widening of the incision trench, rather than pond formation, it is important to recognize that both primary and secondary dams may achieve similar objectives if they experience a breach. Both a mid-structure breach and/or an end-cut (where the structure is undermined along the bank) create hydraulic jets analogous to those purposefully created by a constriction dam. Therefore, while the principal objective of primary and secondary BDAs is not sediment recruitment or incision trench widening, it is important to recognize that this may still be treated as a lesser objective, whereby anticipated structure failure provides benefits.
In areas where beaver dams are present (actively maintained or abandoned) adding fence-posts can increase the dam’s longevity. Continued dam failure will often cause beavers to abandon a location before the colony can establish a stable complex. Reinforcing intact dams with wooden posts reduces the likelihood of dam failure which extends the life-span of individual structure and increases the chances of beaver persistence and/or reoccupation of the area potentially leading to the creation of new dams/complexes. Reinforcing existing dams does not involve construction of new structures so the principle consideration is deciding whether or not to reinforce a particular dam. This decision should be based on:
Reinforcing existing structures relies on installing untreated wooden posts directly downstream of the dam crest or within structural gaps/breaches within the dam. Depending on the condition of the structure woody vegetation and/or sediment may be required to force upstream ponding.