4. PALS and BDAs

Chapter 4: Mimicking and Promoting Wood Accumulation and Beaver Dam Activity with Post-Assisted Log Structures and Beaver Dam Analogues

The fourth chapter is meant to be the authoritative reference on two low-tech restoration techniques, which are lacking guidelines in the literature: post-assisted log structures (PALS) & beaver dam analogues (BDAs). The chapter focuses on helping readers understand what PALS & BDAs are, how they are used in low-tech restoration design, and a specific focus on the processes intially mimicked, later promoted, and that eventually become self-sustatining; namely wood accumulation and beaver dam activity.

Chapter 4 also has appendices on:

  • Appendix A. Freqently Asked Questions About PALS and BDAs
  • Appendix B: Low-Tech Process-Based Restoration Semantics
  • Appendix C: Recent History of Beaver Dam Analogues
  • Appendix D: Typical Schematics and General Suggestions for Placement and Construction of PALS
  • Appendix E: Typical Schematics and General Suggestions for Placement and Construction of BDAs
Download Chapter 4

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Implications for Practice

  • Post-assisted log structures (PALS) and beaver dam analogues (BDAs) are hand-built structures. PALS mimic and promote the processes of wood accumulation; whereas BDAs mimic and promote beaver dam activity.
  • PALS and BDAs are permeable, temporary structures, built using natural materials.
  • BDAs differ from PALS in and that BDAs create ponds using a variety of fill materials; PALS are built with only woody material, which tends to be larger diameter than the woody material used for BDAs.
  • PALS and BDAs are both intended to address the broad impairment of structural starvation in wadeable streams, but can also be used to mitigate against a range of more specific impairments.
  • PALS and BDAs can be built using a variety of natural materials, and built to a range of different shapes, sizes and orientations.
  • PALS and BDAs are most likely to achieve restoration goals when built in high numbers.
  • Some PALS and BDAs are likely to breach and/or lose some wood, but when many structures are installed, that material will accumulate on downstream structures or in natural accumulation areas leading to more complexity.

Beaver Dam Analogue Recipes Post-Assisted Log Structure Recipes
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