Cheap and Cheerful Stream Restoration

Beaver Dam Analogs

What is a Beaver Dam Analog?

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.

Examples of Beaver Dam Analogs

What is a BDA Complex?

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.

BDAs are simple

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.

BDAs are cost-effective

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 are non-destructive

BDAs mimic natural beaver dams, as such they require minimal disturbance of the channel, and no heavy machinery.

What can BDAs do?

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

Beaver Dam Analogs: Primary dams, secondary dams and constriction dams

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: Function, Design and Construction

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:

  • Provide immediate deep-water cover to promote beaver colonization and provide flow refugia and thermal refugia for fish
  • Increase the frequency, duration, and extent of overbank flows ( i.e., increase channel-floodplain connectivity)
  • Increase water table elevation by overbank recharge and lateral, subsurface recharge
  • Capture sediment and cause aggradation and increase channel-floodplain connectivity by increasing channel bed elevation

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:

  • Incision trench width – wider streams allow for greater pond area and generally have higher width-to-depth ratios which reduces the likelihood of dam breaching during high flow events
  • Channel gradient – lower gradient segments allow greater upstream ponding area and are less prone to breaching
  • Incision height/presence of inset floodplain – the vertical distance between the floodplain and/or terrace and channel bed and the flow regime determines whether BDA structures can cause inundation of those surfaces and overbank flows/flow dispersion
  • High flow channels – active or historic high flow (floodplain) channels can be used to reactivate important habitat and deliver flow across the floodplain during high flow events
  • Bank stability – banks that are more resistant to erosion due to armoring and/or vegetation are less likely to experience erosion and breaching by end cuts.

Dam crest elevation (dam height) is determined by a combination of local channel morphology and restoration objectives, including:

  • Desired channel bed elevation – the dam crest elevation represents the maximum bed elevation to which the channel can aggrade
  • Elevation of benches, floodplains – dam crest elevations higher than adjacent surfaces will cause immediate ponding on those surfaces while crest elevations slightly below will only cause inundation during high flow events
  • Channel width-to-depth ratio – channels with a low width to depth ratio may require a lower dam crest elevation to prevent dam breaching during high flows.

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

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:

  • Increase primary dam stability by providing gradient control – increase water surface elevation downstream of the primary dam which decreases the hydraulic gradient, decreasing likelihood of headward erosion/scour
  • Capture return flow – overbank flow caused by primary dams can be recaptured in secondary dams provided flows return to the channel upstream of the secondary dam

Objectives shared by secondary dams and primary dams include:

  • Pond formation – smaller in extent than those caused by primary dams, however can still provide flow refugia and thermal refugia for fish and habitat for beaver colonization
  • Increase geomorphic complexity – pond formation upstream and scour pool and bar formation downstream of structure
  • Increase aggradation – ponding causes aggradation and can increase channel-floodplain connectivity

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:

  • Gradient control – dams that provide gradient control should cause ponding that reaches the base of the upstream structure while also maximizing the areal extent of ponding. The distance downstream from the primary structure will vary based on channel gradient but is generally within 1-2 channel widths of the primary structure.
  • Return flow capture – dams that are intended to capture return flow need to be located downstream of the re-entry point of overbank flows. Location is therefore site specific. Examining the site for overbank channels, and the presence of benches, inset floodplain, terraces and other relevant geomorphic features can help identify potential points of re-entry
  • Geomorphic/habitat complexity – dams designed to increase habitat complexity should be located far enough downstream of the primary structure such that ponding does not reach the primary structure. This will maximize hydraulic and geomorphic complexity by allowing scour pool and bar formation between structures.

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.

  • Dam profile – dam profiles can be straight as opposed to convex, since flow dissipation is less of a concern on structures with lower crest heights. (Can be convex if structure stability is a concern.)
  • Base stabilization – a less extensive base of sediment is required
  • Mattress construction – less extensive/no mattress is required

Constriction 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:

  • Sediment recruitment – flow can be directed at erodible banks in order to mobilize sediment for bar development and aggradation behind dams
  • LWD recruitment – LWD can be recruited by eroding banks where LWD is present
  • Scour pool/bar creation – where banks are resistant to erosion, flow constriction can cause the formation of a scour pool
  • Widening of channel/incision trench – bank erosion increases the width of the incision trench in incised settings, thereby decreasing specific stream power and promoting BDA persistence
  • Increase channel length – bank erosion increases sinuosity and channel length which reduces channel slope and stream power, decreasing the probability of BDA failures.
  • Bank erosion prevention – flow can be directed away from erodible banks to protect infrastructure

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:

  • Existing constriction points – constriction dams can enhance existing flow convergence caused by structural elements (e.g. boulders and LWD) or in meander bends
  • Reach gradient – reaches with higher gradient will have greater potential to do geomorphic work
  • Percent channel constriction – a higher portion of the channel (e.g. ~90%) should be constricted when stream power is low and/or when the desired geomorphic response is high. Along streams with greater stream power desired geomorphic outcomes may be accomplished with less constriction e.g. 50-70% constriction.
  • Erodible/non-erodible features – erodible and erosion resistant boundaries will lead to different geomorphic responses to constrictions i.e. scour pool and bar formation or bank erosion and increase in sinuosity. Constriction dams designed to promote lateral movement of the channel should avoid banks that are resistant to erosion due to armoring and/or vegetation.
  • Erosion-resistant structural elements – in-channel and bank-attached boulders and LWD can be incorporated into the structure to improve stability
  • Bank textural composition – consider grain size that will be recruited by bank erosion i.e. coarse vs. fine sediment

Construction of constriction dams relies on similar methods used to construct both primary and secondary dams, with a few important differences:

  • Dam profile – straight and oriented ~ 120 downstream
  • Not channel-spanning - span 50-90% of the channel
  • Post-line – often two lines of posts are used in order to increase the stability of the structure and allow for more fill material
  • Fill material – High amounts of sediment are used to decrease the permeability of the structure to ensure all flow is forced to the constriction point

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.

Reinforce Existing Dams

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:

  • Condition of existing dam – some portion of the dam should be intact
  • Evidence of current/past hydrologic and geomorphic influence – does the dam cause overbank flows, downstream pool/bar formation?
  • Presence of suitable forage – prioritize sites with sufficient forage to encourage beaver persistence/recolonization
  • Function within a complex – integrate reinforced dam with the greater complex goals (see next section

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.