Increasing channel-floodplain connectivity is often a goal of river restoration. We define connectivity as the ability of energy and materials to move between different areas on the landscape. In river restoration that often means water, sediment, nutrients and wood may move from the channel to the floodplain and vice-versa. Channel-floodplain connectivity therefore is controlled by the interaction of two factors: 1) channel geometry and 2) flow regime. Channel-floodplain connectivity can therefore by reduced by channel incision that prevents flows from being able to overtop banks, or by diminished flows from upstream flow regulations.
BDA can be used to address both causes of decreased floodplain connectivity. BDAs cause aggradation that increases the elevation of the channel bed, reducing the vertical distance to the floodplain that can enable flows to reach floodplain during higher flow events. (The amount and rate of aggradation depends on local and watershed factors such as sediment supply. Also, depending on the magnitude of incision channel-floodplain connectivity may take multiple years to re-establish.) By ponding water and adding roughness to the channel BDAs also increase stage during low and high flow events which increases the likelihood of overbank flows. In areas with limited incision, BDAs can be built to cause immediate floodplain connectivity.
BDAs are not intended to be permanent structures. Like beaver dams, BDAs may be breached during high flow events, the outcome of a breach depends on how the BDA is breached, the type of BDA and the local geomorphic setting. BDAs may breach in the center of the structure by overtopping or along the bank by endcuts. The type of breach therefore controls the local geomorphic response; overtopping can result in a scour pool below the structure, while endcuts promote bank erosion, channel widening and an increase in sinuousity.
The amount of sediment evacuated by a dam breach depends on the local geomorphic setting and the nature of the breaching event. At the individual structure scale (~ 101 m) the amount of sediment evacuated will depend on the nature of the breach (endcut or mid-structure), the magnitude of the flow event, and local geomorphic factors like channel slope and confinement. Because BDAs are clustered into complexes sediment evacuated from one BDA is not delivered from the reach unless all BDAs are breached in unison. Instead sediment evacuated from one BDA will be stored behind downstream BDAs or on inset floodplain surfaces where flow is forced overbank by downstream BDAs.
The existence of broad lowland alluvial valleys formed of fine sediment attributable to beavers suggests that much sediment is retained even following the extirpation and or removal of beaver. Previous studies have found variable rates of sediment evacuation following the breaching of beaver dams and suggested that local hydrologic and geomorphic controls determine the amount of sediment stored following breaching.
Yes, BDAs can be explicitly designed to encourage bank erosion in highly incised streams characterized by low sinuosity. In those situations increasing incision trench width may be necessary before attempting to promote aggradation. It is very important to conceptualize BDA restoration at the complex level. While individual structures may cause aggradation and/or degradation the goals are at a complex level, which tends to be a reach scale on the order of 102 m. At this scale unless specifically designed to cause erosion, aggradation is the dominant response.
The number of BDAs constructed is determined by the restoration objectives. Depends on objectives and setting. Required for what? Increasing complexity or maximizing water storage, promoting growth of riparian zone etc.?
Yes. Locally, site selection is driven by a structure’s ability to achieve restoration objectives, therefore where high flow channels are accessible, or reconnection to the floodplain can happen. Also, more broadly, they should be used in areas where beaver could generally hang out- this tends to mean lower gradient, while beaver can build dams at slopes up to 6%, and in confined settings, those settings are source areas (Schumm) and the process domain is characterized by high hillslope connectivity, debris flows, landslides etc., in short we wouldn’t be maximizing our restoration efforts in a such a disturbance regime.
Yes. BDAs can serve a number of purposes. In areas where there are no beaver they can create immediate ponding for beavers to escape predation. They can reinforce dams that breach regularly, (why dams breach is addressed in the question “do BDAs address the root causes of degradation?). Bridge, for example, has shown that in places where there were/are beaver but due to external drivers the dams couldn’t persist, that reinforcing them allowed them to persist, and capture the ecological benefits traditionally associated with beaver dams.
Alternatively if there isn’t sufficient dam building material or forage, especially the woody vegetation beaver require for winter food caches then creating conditions to facilitate the growth of a healthy riparian area is critical. BDA’s can increase channel-floodplain connectivity, increase water table elevation and increase geomorphic complexity that manifests as bars that are required for many riparian species to grow. Only after sufficient regeneration of the riparian area should beaver be reintroduced-we’re talking long term.
It is therefore important to note any restoration project is held to this same limitation, regrowth of riparian zones is critical, where heavy machinery is used there needs to be a plan to promote immediate regrowth as well as making sure that processes are intact that will promote continued riparian health.
Woody vegetation is necessary. Forage for beaver is necessary if beaver are going to beintroduced. However BDA’s have utilized other woods.
How many BDAs should we build?
BDAs are designed to be implemented in areas that historically had beaver populations, and as such are generally located in partially confined or unconfined valley settings. These settings are characterized by medium to low hillslope connectivity and medium to high floodplain development. Similar to beaver (and this is a major consideration where beaver reintroduction is an objective) BDAs are designed to be implemented in areas that are conducive to their persistence. While they are not intended to be permanent structures they are designed to persist over timescales consistent with long-lived beaver dams (~101 years).
Therefore highly confined, high gradient streams are not the intended setting for BDAs. Similarly rivers with high annual peak flows, incapable of being dammed by beaver are not the intended setting for BDAs.
Much restoration literature emphasizes the importance of addressing the root causes of degradation. In a fluvial setting the causes of degradation can be local (e.g., channel straightening, levees, dams) or widespread (e.g., deforestation, urbanization, agriculture). Also degradation can be caused by actions that have both systemic and local effects such as the removal of riparian vegetation. In many cases the initial causes of degradation constrain the processes that are essential for functioning riverine ecosystems, or may have moved the stream into an alternative stable state incapable of supporting important stream functions and processes.
Clearly BDAs do not address deforestation, dam building or the removal of riparian vegetation. However, in each of those cases, there are specific (often multiple) affected that have resulted in the new condition. BDAs are intended to influence the processes initially affected by previous actions and create local and reach scale conditions that can restore processes that are critical to riverine health.
Where the removal of beaver is one of the causes of degradation than BDAs do facilitate and/or mimic reversing the precise cause of degradation. Where riparia have been removed, channel incision has taken place, or channel straightening (note that all three can be caused by multiple stressors) than BDAs can influence the hydraulic processes that were affected. Regarding riparia, BDAs can create the hydrological conditions (overbank flows, increase water table elevation) to recover the hydrology necessary to restoring riparian habitats. Regarding channel incision, BDAs promote channel aggradation and reconnection to the floodplain. Regarding channel straightening, BDAs can induce meanders and create and inset floodplain.
BDAs are not intended to be universally applicable.
Many of the risks associated with BDAs
The maintenance required for BDAs depends on annual flow events (stochastic) and whether or not beaver are present. If beaver are present and maintaining BDAs (or alternatively building new dams) then little or no maintenance may be required. If beaver are not present, seasonal maintenance will likely be required to maintain ponding. Depending on the condition of the structure maintenance can include adding additional posts, weaving woody vegetation and/or patching small gaps using cobbles and sediment.