Low-Tech Process-Based Stream Restoration
Using Post-Assisted Log Structures (PALS) and Beaver Dam Analogues (BDAs)
Beaver dams slow the flow of water in stream channels. This decreases erosion and deposits suspended particles in the impoundments. Unfortunately, centuries ago unregulated trapping eliminated most beavers and their dams. Without beaver dams to slow, sink and store stream water, many North American streams became badly eroded, incised and damaged.
As erosion cuts a stream channel deeper and deeper into the ground, the surrounding water table also drops. Once the water table drops too low for the roots of nearby plants to reach, the lush native vegetation dies off resulting in a barren, arid landscape with a loss of biodiversity. Without beaver dams, a lush lowland can become a desolate, dust bowl.
It is well known that beaver dams reverse the damage of stream incision. However, some incised channels are so steep and the vegetation so barren that beavers cannot live there. Fortunately though, we can undo some of this damage and entice beavers back to these severely damaged streams by building Beaver Dam Analogs (BDA’s) and Post-Assisted Log Structures (PALS). Research has shown that these manmade in-stream structures promote sediment deposition. The rapidity of sediment deposition and the corresponding water table rise by these simple manmade structures is impressive. BDA’s and PAL’s reverse stream incision, and can reconnect a stream to its floodplain. Once in place, a single-thread, fast flowing channel will develop complexity, creating a variety of habitats that support more life.
The use of BDA’s, PAL’s and similar structures to heal incised stream is often called low-tech, process-based restoration (LTPBR). An excellent LTPBR manual can be downloaded HERE.
Similar to natural beaver dams, LTPBR slows water velocity, allowing suspended particles to settle. As sediment accumulates, the stream bottom rises resulting in corresponding elevations of the surrounding water table. Stream banks become less steep and the stream once again becomes hydrologically connected to the floodplain. The higher water table allows native shrubs and trees to return which can then support beavers. Beavers continue the stream restoration process by building their own dams, often on top of the these wooden structures. The riparian corridor becomes a lush and healthy ecosystem once again and attracts a multitude of species. Biodiversity flourishes, and the stream restoration process is completed.
The beauty and increasing popularity of using low-tech, process-based restoration for streams includes their low cost and high success rate. BDA’s and beavers effectively restore severely damaged stream corridors, and they do it at a much lower cost and higher success rate than humans alone could accomplish.
Beaver Restoration Guidebook v. 2.0
The definitive reference for beavers and stream restoration.
Low-Tech Process-Based Restoration of Riverscapes – Design Manual, Utah State Restoration Consortium
Building Beaver Dam Analog Videos, Utah State Univ.
Low-Tech Process-Based Restoration Construction Videos, Utah State Univ.
Pocket Guide to BDA’s, Utah State Univ.