A BDA complex is typically composed of 2-8 individual structures that are designed to work together to achieve restoration goals. Because restoration goals are defined at the reach and watershed scale, complexes are the minimum scale at which restoration goals should be articulated.
BDA complexes can be specifically designed to meet specific restoration goals by manipulating the spatial arrangement of different types of BDAs (i.e., the number and order of primary, secondary, and constriction dams). Designing a complex to meet a specific restoration objective is the natural result of restoring rivers that are in different states of degradation and at different points in their channel evolution/trajectory. For example, BDA complex design will differ between highly incised, low sinuosity channels with low riparian vegetation and inset floodplains and channels with moderate incision, moderate sinuosity and higher riparian vegetation and inset floodplain. In many cases the restoration goals are not mutually exclusive, and are in fact inextricably linked (e.g., overbank flows and habitat complexity and riparian vegetation and groundwater recharge).
The assumption underlying all of the practices outlined here is that if beaver can be reintroduced/supported their ability to achieve restoration goals is far greater than our own. In other words, our objective is to help beaver get established and then get out of the way. With this in mind, we recognize that while we may build a BDA complex to address certain objectives if beaver are successfully reintroduced they will likely build their own dams and may not maintain initial BDAs. In some cases they may even build dams immediately downstream of existing BDAs, inundating them in the process.
The remainder of this section provides examples of how BDA complexes can be designed to meet specific restoration goals. Because the restoration of highly degraded streams may require multiple treatments the following section outlines how different BDA complexes could be used in succession to meet overall restoration objectives.
Setting/Condition Moderately (~1-2 m) incised channel, low sinuosity, no inset floodplain or riparian vegetation.
Restoration objective of Phase I Primary objective: Widen incision trench, create inset floodplain, Secondary objective: Increase geomorphic complexity. Rationale In highly incised settings, without accessible floodplain surfaces or riparian vegetation, reintroducing beaver is inappropriate (no food). Furthermore at higher discharges, in the absence of accessible floodplains unit stream power increases significantly which increases the probability of dam breaching. In order to create the conditions that would allow for riparian development surfaces must first be created. Widening the channel promotes lower unit stream power by increasing channel width and lower gradient by increasing sinuosity, both of which create conditions conducive to deposition.
Complex Design This complex is relies heavily on constriction dams that increase bank erosion in order to widen the channel and provide sediment that can be redistributed as in inset floodplain. Channel spanning dams are used to increase geomorphic complexity by forcing pools and creating scour pools and bar deposition. They may also act to capture some sediment eroded by upstream constriction dams. In some instances these dams may be breached during high flows. Channel spanning dams can be build to breach along the banks (rather than mid-channel) to cause additional widening.
Moderate incision (~1-2 m), moderate sinuosity (increased from phase I), inset floodplain and bars present, low amounts of riparian vegetation.
Restoration objective of Phase II Primary objective: Aggrade channel bed, increase geomorphic complexity Secondary objective: Promote riparian regrowth Rationale Lack of woody riparian vegetation prohibits beaver reintroduction. Moderate channel width and presence of inset floodplain suggest that it is possible to cause aggradation (eventual reconnection to floodplain), and that BDA persistence is higher as high flows will be able to disperse over newly formed inset floodplain. Depositional and erosional surface provide colonization sites for riparia where they may maintain contact with the water table.
This complex is composed predominantly of channel spanning dams that capture sediment in order to increase aggradation, and force flow onto the existing inset floodplain. Some constriction dams may be used to create a sediment source for aggradation if natural sediment supply is low. Channel spanning dams continue to increase pool habitat by forcing upstream ponding and the formation of scour pools downstream. Riparian planting may be important if there are no local source populations. Channel spanning dams increase water table elevation by increasing flow onto inset floodplain and lateral subsurface recharge, creating conditions favorable for riparia.
Low incision (~0.5-1 m), moderate sinuosity, inset floodplain with moderate amount of riparian growth
Restoration objective of Phase III Primary objective: increase channel aggradation to reconnect to floodplain, increase riparian vegetation, increase geomorphic complexity
Rationale Lack of sufficient woody riparia prohibits beaver reintroduction. Phase III is largely an extension of phase II with a high starting bed elevation that makes infrequent inundation of formerly abandoned floodplain possible. Increasing riparia is a major objective now since it is the most significant constraint on reintroducing beaver.
Similar to phase II, this complex relies on a series of channel spanning dams that capture sediment in order to aggrade the channel bed and force overbank flows during high flow events. Ponds maintain a higher water surface allowing for lateral subsurface groundwater recharge. High flows are now spread out over floodplain that has been colonized by riparian vegetation which increases roughness, promoting deposition, and increasing stability. Because riparian vegetation is now the major constraint on beaver reintroduction, practices that promote riparian growth such as fencing or grazing regime are major considerations. (These practices can have been implemented sooner. They also may not be required in all settings.)
Negligible incision, moderate to high sinuosity, moderate-high riparian vegetation.
Restoration objective of Phase IV Primary objective: Reintroduce Beaver Secondary objective: maintain geomorphic complexity and riparian vegetation
Rationale Adequate hydrology and vegetation to support beaver reintroduction. If beaver reintroductions is unsuccessful however, do not want to lose the benefits gained to this point, therefore some construction of maintenance of BDAs is required.
Complex Design This complex is composed entirely of channel spanning dams. Because the goal is to encourage beaver reintroduction creating extensive and deep pool used by beaver is the main objective. Therefore primary dams that maximize pond extent are prioritized. Other considerations include placement of structure relative to cover, forage and building material. Secondary dams are used as support structures for primary dams and therefore should pond water to the base of the primary dam.
In the event that the effort to reintroduce beaver to the restoration site is unsuccessful at first (due to no beavers to move, predation, emigration or other) maintaining BDAs is important to maintaining pond habitat, geomorphic complexity and hydrology that supports riparian vegetation with the hopes that future reintroduction efforts will be successful.