Michael M. Pollock

The Network Dynamics Hypothesis: How Channel Networks Structure Riverine Habitats

Abstract Hierarchical and branching river networks interact with dynamic watershed disturbances, such as fires, storms, and floods, to impose a spatial and temporal organization on the nonuniform distribution of riverine habitats, with consequences for biological diversity and productivity. Abrupt changes in water and sediment flux occur at channel confluences in river networks and trigger changes in channel and floodplain morphology. This observation, when taken in the context of a river network as a population of channels and their confluences, allows the development of testable predictions about how basin size, basin shape, drainage density, and network geometry interact to regulate the spatial distribution of physical diversity in channel and riparian attributes throughout a river basin. The spatial structure of river networks also regulates how stochastic watershed disturbances influence the morphology and ages of fluvial features found at confluences.

A Review of Stream Restoration Techniques and a Hierarchical Strategy for Prioritizing Restoration in Pacific Northwest Watersheds

Abstract Millions of dollars are spent annually on watershed restoration and stream habitat improvement in the U.S. Pacific Northwest in an effort to increase fish populations. It is generally accepted that watershed restoration should focus on restoring natural processes that create and maintain habitat rather than manipulating instream habitats. However, most process-based restoration is site-specific, that is, conducted on a short stream reach. To synthesize site-specific techniques into a process-based watershed restoration strategy, we reviewed the effectiveness of various restoration techniques at improving fish habitat and developed a hierarchical strategy for prioritizing them. The hierarchical strategy we present is based on three elements: (1) principles of watershed processes, (2) protecting existing high-quality habitats, and (3) current knowledge of the effectiveness of specific techniques. Initially, efforts should focus on protecting areas with intact processes and high-quality habitat. Fol...

Process-based Principles for Restoring River Ecosystems

Process-based restoration aims to reestablish normative rates and magnitudes of physical, chemical, and biological processes that sustain river and floodplain ecosystems. Ecosystem conditions at any site are governed by hierarchical regional, watershed, and reach-scale processes controlling hydrologic and sediment regimes; floodplain and aquatic habitat dynamics; and riparian and aquatic biota. We outline and illustrate four process-based principles that ensure river restoration will be guided toward sustainable actions: (1) restoration actions should address the root causes of degradation, (2) actions must be consistent with the physical and biological potential of the site, (3) actions should be at a scale commensurate with environmental problems, and (4) actions should have clearly articulated expected outcomes for ecosystem dynamics. Applying these principles will help avoid common pitfalls in river restoration, such as creating habitat types that are outside of a sites natural potential, attempting to build static habitats in dynamic environments, or constructing habitat features that are ultimately overwhelmed by unconsidered system drivers.

The Importance of Beaver Ponds to Coho Salmon Production in the Stillaguamish River Basin, Washington, USA

Abstract The use of beaver Castor canadensis ponds by juvenile coho salmon Oncorhynchus kisutch and other fishes has been well established. However, the population-level effects on coho salmon resulting from the widespread removal of millions of beaver and their dams from Pacific Coast watersheds have not been examined. We assessed the current and historic distributions of beaver ponds and other coho salmon rearing habitat in the Stillaguamish River, a 1,771-km2 drainage basin in Washington and found that the greatest reduction in coho salmon smolt production capacity originated from the extensive loss of beaver ponds. We estimated the current summer smolt production potential (SPP) to be 965,000 smolts, compared with a historic summer SPP of 2.5 million smolts. Overall, current summer habitat capacity was reduced by 61% compared with historic levels, most of the reduction resulting from the loss of beaver ponds. Current summer SPP from beaver ponds and sloughs was reduced by 89% and 68%, respectively, co...

Ecosystem experiment reveals benefits of natural and simulated beaver dams to a threatened population of steelhead (Oncorhynchus mykiss).

Beaver have been referred to as ecosystem engineers because of the large impacts their dam building activities have on the landscape; however, the benefits they may provide to fluvial fish species has been debated. We conducted a watershed-scale experiment to test how increasing beaver dam and colony persistence in a highly degraded incised stream affects the freshwater production of steelhead (Oncorhynchus mykiss). Following the installation of beaver dam analogs (BDAs), we observed significant increases in the density, survival, and production of juvenile steelhead without impacting upstream and downstream migrations. The steelhead response occurred as the quantity and complexity of their habitat increased. This study is the first large-scale experiment to quantify the benefits of beavers and BDAs to a fish population and its habitat. Beaver mediated restoration may be a viable and efficient strategy to recover ecosystem function of previously incised streams and to increase the production of imperiled fish populations.

Using reference conditions in ecosystem restoration: an example for riparian conifer forests in the Pacific Northwest

Quantifying the attributes of reference sites is a crucial problem in the restoration of ecosystems, driving both the evaluation of current conditions and the setting of management targets for specific points in the future. Restoration of riparian ecosystems, particularly those dominated by conifers, has become a priority because of the numerous ecosystem services they provide, including a high number of vertebrate species in population decline that utilize these structurally complex forests. By way of example, we illustrate a three-step process to assess the effects of proposed riparian ecosystem restoration efforts: (1) identify reference sites (2) quantify metrics that describe the reference sites, and (3) use models to predict the likely effects of restoration actions relative to reference conditions. To this end, we identified 117 natural, late-successional conifer dominated stands from existing forest inventories in the Pacific Northwest for the purpose of establishing reference conditions. We did this to establish quantitative metrics for structural attributes essential to the maintenance of biodiversity in these forests, and to assess whether there were any important quantitative differences between upland and riparian forests or whether upland and riparian forest reference sites could be used interchangeably. Both forest types were generally similar, but riparian stands had higher average live tree wood volumes and basal areas, suggesting they may be growing on sites that are more productive. Both riparian and upland forests had abundant large diameter (>50 cm) live trees and snags. Collectively, our data suggest that mature, late-successional conifer dominated forests have well developed structural characteristics in terms of abundant large trees in the overstory, abundant large snags, and a well-developed understory of shade-tolerant trees. We modeled the growth of young conifer stands to assess whether a common restoration treatment would accelerate development of structural characteristics typical of reference conditions. We found that left untreated, the stands followed a trajectory towards developing forest structure similar to the average reference condition. In contrast, the restoration treatment followed a developmental trajectory along the outside range of reference conditions.

Restoring Rivers in the Twenty-First Century: Science Challenges in a Management Context

Abstract Legal mandates force consideration of at least some level of river restoration in many developed nations (e.g., Clean Water and Endangered Species Act in the United States, or the Water Framework Directive in the European Union), but a lack of specifics in legislation compels decision-makers to ask three persistent management questions: (1) How much river restoration do we need? (2) How do we best achieve cost-effective river restoration? (3) How do we know we have restored enough? Moreover, the broader management context is permeated with tremendous inertia to continue development of rivers for societal and economic gain, continual application of small and fragmented restoration actions, and skepticism that river restoration can succeed in the face of climate change and steady population growth. It is in this context that we identify key science challenges for river restoration in the twenty-first century. We suggest that a fundamental shift toward restoring watershed and river processes (process-based restoration) is needed if scientists are to begin developing the tools needed to provide relevant policy answers. The basic conceptual framework of process-based restoration requires that we understand how habitat is formed and changes, how habitat changes alter biota, and how human actions alter both river habitats and the landscape processes that create river habitats. Restoration actions must then directly address human actions that caused habitat degradation, thereby addressing the root causes of biological impacts. Understanding this framework will allow scientists to better address key science challenges for advancing river restoration, including development of eco-system models to predict what kinds of and how much restoration is needed, an expanded suite of process-based restoration techniques for large river ecosystems, and comprehensive but cost-effective suites of metrics for monitoring river health.

Methods for Successful Establishment of Cottonwood and Willow Along an Incised Stream in Semiarid Eastern Oregon, USA

Restoration of riparian plant communities in the dry interior Columbia River basin and other semiarid regions where channel incision has altered hydrological connections is a major challenge. Establishing riparian vegetation within incised systems has the potential to facilitate the aggradation of the stream bed by increasing bank stabilization and providing large woody debris inputs. We tested an alternative approach to irrigation in a heavily incised subwatershed of the dry interior Columbia River Basin by using motorized augers to penetrate lowered water tables. By planting dormant pole cuttings of black cottonwood (Populus trichocarpa) and willow (Salix spp.) in auger holes within terraces up to 2 m above the incised streambed elevation, we hypothesized that riparian vegetation could be established without the aid of irrigation. We also tested the efficacy of vented plastic tree shelters and circular fence cages with respect to survival, growth, and browsing after 15 months. Survival and average growth was significantly higher for pole plantings protected with vented plastic tree shelters and planted in auger holes where lowered water tables were penetrated (89% and 95 cm, respectively). Survival and average growth was significantly lower for unprotected pole cuttings planted in augur holes where lowered water tables were not penetrated (5% and 33 cm, respectively). Our results suggest that survival of willow and cottonwood pole plantings along incised streams can be greatly enhanced if they are: 1) placed in auger holes that reach the water table via deep-planting; and 2) protected with vented plastic tree shelters.

Modeling intrinsic potential for beaver (Castor canadensis) habitat to inform restoration and climate change adaptation

Through their dam-building activities and subsequent water storage, beaver have the potential to restore riparian ecosystems and offset some of the predicted effects of climate change by modulating streamflow. Thus, it is not surprising that reintroducing beaver to watersheds from which they have been extirpated is an often-used restoration and climate-adaptation strategy. Identifying sites for reintroduction, however, requires detailed information about habitat factors—information that is not often available at broad spatial scales. Here we explore the potential for beaver relocation throughout the Snohomish River Basin in Washington, USA with a model that identifies some of the basic building blocks of beaver habitat suitability and does so by relying solely on remotely sensed data. More specifically, we developed a generalized intrinsic potential model that draws on remotely sensed measures of stream gradient, stream width, and valley width to identify where beaver could become established if suitable vegetation were to be present. Thus, the model serves as a preliminary screening tool that can be applied over relatively large extents. We applied the model to 5,019 stream km and assessed the ability of the model to correctly predict beaver habitat by surveying for beavers in 352 stream reaches. To further assess the potential for relocation, we assessed land ownership, use, and land cover in the landscape surrounding stream reaches with varying levels of intrinsic potential. Model results showed that 33% of streams had moderate or high intrinsic potential for beaver habitat. We found that no site that was classified as having low intrinsic potential had any sign of beavers and that beaver were absent from nearly three quarters of potentially suitable sites, indicating that there are factors preventing the local population from occupying these areas. Of the riparian areas around streams with high intrinsic potential for beaver, 38% are on public lands and 17% are on large tracts of privately-owned timber land. Thus, although there are a large number of areas that could be suitable for relocation and restoration using beavers, current land use patterns may substantially limit feasibility in these areas.

Evaluation of dryland riparian restoration with cottonwood and willow using deep‐planting and herbivore protection

Degradation of dryland riparian ecosystems has been linked to the lowering of alluvial groundwater tables and reduced floodplain connectivity. Establishing riparian plants in dryland ecosystems with high water-stress and herbivore pressure presents major challenges for restoration practitioners. By planting at sufficient depths to reach lowered water tables, deep-planting provides direct access to water and encourages root development within hydrated soils. While deep-planting is a promising alternative to traditional supplemental irrigation in dryland areas affected by lowered water tables, few studies have evaluated deep-planting where planting depths must exceed one-meter to reach water tables and where herbivore protection is required. To evaluate deep-planting as an irrigation alternative where lowered water tables present a challenge to riparian restoration, we conducted experimental plantings along an incised stream within a semiarid watershed using deep-planting without supplemental irrigation in ...