Process-based restoration (PBR) is an approach to the ecological restoration of degraded watersheds and riverscapes that aims to reestablish keystone processes, such as hydrology, sediment transport, carbon sequestration, nutrient cycling, and fire cycle resiliency.
When carrying out PBR in riparian systems, a fundamental question we ask ourselves is how we can harness natural energy sources to accelerate the pace and scale of river, wetland, and floodplain restoration. In designing these treatments, we consider how we can leverage four key elements: space, materials, energy, and time (see illustration below) and what source problems are keeping us from taking full advantage of these key agents of change.
We also consider the following Riverscape Principles laid out by our colleagues at Utah State University (excerpted from their excellent Low-Tech Process Based Restoration of Riverscapes Pocket Guide):
- Streams need space
- Structure forces complexity and builds resilience
- The importance of structure varies
- Inefficient conveyance of water is often healthy
- It is okay to be messy
- There is strength in numbers
- Use natural building materials
- Let the system do the work
- Defer decision-making to the system
- Self–sustaining systems are the solution
Beaver as agents of PBR
We look to beaver as agents in process-based restoration who actively shape landscapes and promote natural ecological processes. Beaver dams and associated ponds help mitigate the impacts of climate change, drought, and wildfire by sequestering carbon, increasing surface and groundwater storage, attenuating flood waters, and creating wildfire-resistant riverscapes that serve as critical “refugia” for species unable to escape fire. Beaver dams improve water quality, repair eroded channels, reconnect streams to their floodplains, and create and maintain wetland and riparian habitats.
Beaver create habitat complexity and diversity in otherwise simplified stream systems and can prolong critical summer streamflow or provide perennial flow to degraded urban and wildland streams that would otherwise run dry. Beaver dams, canals, burrows, and food caches greatly expand wetland and wet meadow habitats, create refugia from predation and high flows, and provide many other ecosystem benefits to imperiled fish, birds, and amphibians. Their activities exemplify how a single species can profoundly impact the health and stability of riverscapes, making them invaluable partners in process-based restoration efforts.
Enhancing and/or mimicking beaver
Whether beaver are present or absent humans can act as process-based restoration practitioners. In places where beaver are present, practitioners can reinforce existing dams and add structures between dams to enhance and expand the beaver-managed wetland. Where beaver are not present, practitioners can install structures to mimic a beaver wetland complex. The suite of tools in process-based restoration includes adding woody features such as (much of the following is excerpted from the LTPBR for Riverscapes Pocket Guide. See this guide for more details):
Beaver dam reinforcement
Installing vertically driven untreated posts through an existing beaver dam to enhance its durability and persistence over time.
Riparian revegetation
Planting beaver-favored food and building resources to encourage beaver presence and persistence.
Post-assisted log structures (PALS)
PALS are handbuilt structures that mimic and promote the processes of wood accumulation. Woody material of various sizes pinned together with untreated wooden posts driven into the substrate.
- Bank-Attached PALS: These kinds of PALS have two variations. The first is designed to force a constriction jet and the other is called a “Bank Blaster” which accelerates lateral widening through bank erosion.
- Mid-Channel PALS: Installed mid-channel to split the flow around the structure creating more variation in the flow, pool formation, and sediment deposition.
- Channel-Spanning PALS: Attached to the bank on both sides, such that even at low flow there is some hydraulic purchase across most of the channel, acting to back-water flow behind it. These can have a variable crest elevation and rougher finish and are generally built with much greater porosity.
Beaver dam analogs (BDAs)
BDAs are handbuilt structures that mimic and promote the processes of beaver dam activity. BDAs are permeable, channel-spanning structures with a constant crest elevation, constructed with a mixture of woody debris and fill material to promote temporary ponding of water.
- Postless BDAs: Postless BDA designs are inspired by how beavers build dams; without fence posts, a hydraulic post pounder, or heavy equipment. Like natural beaver dams, the postless BDA is appropriate in areas that can already support beaver dams.
- Post-Assisted BDAs: Posts can provide some temporary anchoring and stability to help with initial dam stability during high flows in systems with flashier flow regimes or that produce larger magnitude floods. Build the BDA first and then add the posts.
- Post and Wicker Line: Posts are used to lay out a crestline, and long branches are woven between the posts to provide most of the structure. post-line wicker weave BDAs have a constant crest elevation so as to not concentrate flow at any point.
Other Forms of PBR
Process-based restoration may also be applied through other land stewardship practices such as planned grazing, prescribed fire, holistic vegetation management, upland gully stuffing, and supplemental riparian and meadow vegetation planting.
At our OAEC Conservation Hydrology demonstration site and other partner sites, we are innovating our own methods of process-based restoration we call, “Fuels to Flows.” This involves using slash from thinning forests of their fuel load reusing this beneficial biomass to slow down water on hillslopes and upland waterways, allowing it to sink in and recharge groundwater, and sequester carbon, while also reducing sediment delivery downstream to critical salmon spawning habitat.
Interventions are guided by a stewardship mentality, whereby they are adaptive over time in response to environmental feedback, with the goal of encouraging a self-sustaining, dynamic ecosystem.
Resources
- Beaver Restoration Guidebook
- LTPBR of Riverscapes: Design Manual
- LTPBR of Riverscapes: Pocket Manual
- Swift Water Design, particularly Think Like a Beaver
- CalPBR Network
- Design Criteria for Process-Based Restoration of Fluvial Systems, Ciotti, et al., 2021
- Let The Water Do The Work: Induced Meandering, And Evolving Method For Restoring Incised Channels
- Groundwork
- Land Stewardship Guide
- Water Stewardship Guide
- OAEC Upland PBR Resources
