With summer upon us, hot weather being the norm in Sunny California, many of us look for any reason to play in the water, even the CDA Project Team. CDA, along with some of our partners, have learned to take advantage of a cool summer project on a hot day, BDA installations. You may not be familiar with terms such as Zuni Pools, One Rock Structures, or BDA’s. That being the case, lets jump in this together.
A Beaver Dam Analog (BDA) is a man-made structure designed to mimic the form and function of a natural beaver dam. 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. In general, the design and installation of BDA complexes is a simple, cost-effective, non-intrusive approach to stream restoration that can influence a suite of hydraulic, geomorphic, and hydrologic processes in order to achieve a range of common restoration goals, not just Beaver relocation. While recipes are helpful when you prepare a new venison dish or meal the first few times, with experience you become more comfortable adapting the recipe to make it your own (never forget the mesquite charcoal). You can substitute ingredients to better match what you might have available in the cupboard (i.e. onsite) and find efficiencies and improvements that work better in your situation.
The photo shows how we were building some BDAs (not all) in a particular setting, for a particular purpose (and with access to plenty of willow to weave between the aspen posts. That simple photo has since appeared in numerous grant applications, though not yet to make its way into ‘engineer’ designed restoration manuals. Lost in those engineered translations has been some of the common sense that any good cook knows. For example, willow weave is not always essential! You can do underwater basket weaving with many different woody materials (so use what is available).
However, the important thing is that these things achieve their desired goals, which is to reduce streambank erosion/incising and restoration of the hydrologic function of meadow systems. Beaver don’t build dams with fence posts, and we don’t always need to either. We often use materials at hand, larger willow, small aspen (usually 3’ or less in diameter) thinned from stands that have been deemed too dense, and juniper to name just a few organic material sources. Smaller structural elements work well such as willows, conifer boughs, and small aspen limbs, along with a good mud packing. These BDAs are not required to remain structurally sound for many years, often their purpose has been accomplished within the first two or three years. Streambank erosion, incising, and channel cut is often greatly reduced within just a few years of constructing the BDA, reducing flows, and often stream beds stabilize and refill to a more functional elevation. We have played with building postless BDAs (particularly in smaller streams with lower peak stream powers) and they have worked remarkably well too.
A bit of advice for the do-it-yourselfer out their looking for some “cool” project time.
Two of the most common mistakes in using BDAs are:
- Focusing too much on the individual structure design. It’s about the collective benefit of many BDAs in a stretch of stream restoration. An Example is our CDA Warner Meadows Project, we put 39 BDAs in two large at risk meadow systems, the area showed incredible gains in a verry short time and the meadow expanded to its historic boundaries in the first year. Hydrological function seemed to reconnect throughout the whole meadow ecosystem.
- Overbuilding BDAs trying to make them too stable (e.g., like an engineered structure). The relative importance of individual dams is lowered when you have several BDAs within a targeted restoration area. Placing a primary dam (larger dam that can support a lodge and often spreads out on to floodplain) strategically within a complex, and then using secondary dams to extend forage access (for beaver), and step down big head drops in smaller steps (by backwater flooding to the base of a taller dam) is far more important. Backwater helps support the upstream structure while reducing high flow currents below. Plus, some failure of individual BDAs is good! Some of the best instream habitat we’ve found is associated with failed beaver dams and beaver dam analogs.
Meadows, riparian zones, and meandering streams, when functioning properly, are some of the most beneficial elements in our ecosystem. Often these areas are vitally important for wildlife, instream invertebrate, and a sundry of migratory species that use these areas for sustenance, bedding, and stopovers. Restoring these high value resources is of great reward and indeed some “cool” summer work for sure.
Team CDA looks forward to seeing you on the mountain soon, and maybe in the stream as well.