The Ecology and Management of Wood in World Rivers

<em>Abstract.</em>—Beaver dams alter the hydrology and geomorphology of stream systems and affect habitat for fishes. Beaver dams measurably affect the rates of groundwater recharge and stream discharge, retain enough sediment to cause measurable changes in valley floor morphology, and generally enhance stream habitat quality for many fishes. Historically, beaver dams were numerous in small streams throughout most of the Northern Hemisphere. The cumulative loss of millions of beaver dams has dramatically affected the hydrology and sediment dynamics of stream systems. Assessing the cumulative hydrologic and geomorphic effects of depleting these millions of wood structures from small and medium-sized streams is urgently needed. This is particularly important in semiarid climates, where the widespread removal of beaver dams may have exacerbated effects of other land use changes, such as livestock grazing, to accelerate incision and the subsequent lowering of groundwater levels and drying of streams.

The Ecology and Management of Wood in World Rivers

<em>Abstract.</em>—Riparian and floodplain forests are vital components of landscapes. They are transitional zones (ecotones) between river and upland ecosystems where ecological processes occurring in riparian areas and floodplains connect and interact with those of rivers and streams. These forests are the major source of large wood for streams and rivers. Extensive loss of riparian and floodplain forests around the globe is evident from the dramatically reduced supply of large wood in rivers. Clearly, it is necessary to conserve and restore riparian forests to sustain a supply of wood for rivers. This chapter discusses river and land management practices that are designed to provide a continuous source of large wood for rivers and retain wood once it has entered the channel or floodplain. These management practices include conservation of intact riparian and floodplain forests, restoration of ecological processes necessary to sustain riparian forests in the long term, and management of riparian forests specifically to accelerate recruitment of large wood to rivers and streams.

The Ecology and Management of Wood in World Rivers

<em>Abstract.</em>—A landscape perspective of wood in world rivers accounts for spatial and temporal patterns of sources of wood from streamside forests, processes of wood delivery to channels, transport of wood through river networks, and trapping sites of wood. Amounts of wood in a river system also depend on productivity of forests in source areas and decomposition rates. Collectively, these factors determine the amount and arrangement of individual pieces and accumulations of wood through a river network, which, in turn, affect ecological, geomorphic, social, and other features of rivers. Research to date deals with subsets of these components of wood in rivers, but there has been limited development of a general framework for wood in river networks. This chapter considers a framework for examining the arrangement of wood in river landscapes and how it may reflect the history of spatial patterns and timing of wood input and redistribution. Field studies provide examples of different spatial patterns and architectures of wood accumulations. Wood accumulations are shaped by input processes, trapping sites, and transport processes. Reaches in river networks may switch from wood patterns dominated by one set of controls to another because of gradual or abrupt input and redistribution. A framework for future studies and management includes interpretation of these different controls through time and over river networks.

The Ecology and Management of Wood in World Rivers

<em>Abstract.</em>—Wood has been falling into rivers for millions of years, resulting in both local effects on channel processes and integrated influences on channel form and dynamics over a wide range of spatial and temporal scales. Effects of stable pieces of wood on local channel hydraulics and sediment transport can influence rates of bank erosion, create pools, or initiate sediment deposition and bar formation. At larger spatial scales, changes in the supply of large wood can trigger changes in both river-reach morphology and the interaction between a river and its floodplain. Over long time scales, wood-rich rivers may retain more sediment and have lower sediment transport rates and steeper slopes than comparable wood-poor channels. Most geomorphic effects of wood in rivers arise from large, stable logs that catalyze changes in the routing and storage of both smaller wood and sediment. The size of a log relative to the channel provides a reasonable gauge of the potential stability of in-channel wood. Channels with a high supply of large, potentially stable wood may experience substantial vertical variability in bed elevation independent from external forcing (e.g., climate variability, temporal variations in sediment supply, or tectonic activity). In some river systems, changes in the wood regime, as described by the size and amount of wood supplied to a river, can result in effects as great as those arising from changes in the sediment supply or the discharge regimes. Consequently, an understanding of the geomorphic effects of wood is crucial for assessing the condition and potential response of forest channels.

The Ecology and Management of Wood in World Rivers

<em>Abstract.</em>—We review published literature examining the role of wood in mediating biodiversity in aquatic ecosystems, identifying the components of biodiversity, taxonomic groups, and scales that have been studied, and highlight gaps in existing knowledge. The components of biodiversity most frequently studied include species diversity (or richness) of macroinvertebrates and fishes, structural complexity within habitat units, and the diversity of habitats found in a stream reach. Many of these studies show that large wood increases biodiversity by providing stable, hard substrates for colonization by periphyton and macroinvertebrates; by increasing microhabitat complexity; and by shaping channel morphology by controlling patterns of erosion and deposition in stream reaches. The abundance of wood in channels, as well as its functional role, varies greatly in longitudinal, lateral, and vertical dimensions along the river corridor. The influence of wood on community structure and ecosystem processes also varies across these dimensions and from stream headwaters to river mouths and nearshore marine environments. Thus, wood can influence biodiversity at all of these scales. Numerous studies, however, have failed to show an effect of wood on biodiversity. These conflicting results illustrate that wood abundance, its functional role in streams, and its influence on biodiversity depend on a variety of factors, and it is the total effect of all these factors, not simply the presence of large wood, that determines patterns of biodiversity.

The Ecology and Management of Wood in World Rivers

<em>Abstract.</em>—Many ecological processes are associated with large wood in streams, such as forming habitat critical for fish and a host of other organisms. Wood loading in streams varies with age and species of riparian vegetation, stream size, time since last disturbance, and history of land use. Changes in the landscape resulting from homesteading, agriculture, and logging have altered forest environments, which, in turn, changed the physical and biological characteristics of many streams worldwide. Wood is also important in creating refugia for fish and other aquatic species. Removing wood from streams typically results in loss of pool habitat and overall complexity as well as fewer and smaller individuals of both coldwater and warmwater fish species. The life histories of more than 85 species of fish have some association with large wood for cover, spawning (egg attachment, nest materials), and feeding. Many other aquatic organisms, such as crayfish, certain species of freshwater mussels, and turtles, also depend on large wood during at least part of their life cycles.

The Ecology and Management of Wood in World Rivers

<em>Abstract.</em>—Although submerged wood obviously influences the flow, little information exists on its various hydraulic effects in streams and rivers. This chapter gives a brief overview of the current knowledge about hydraulic effects of circular cylinders and simple tree shaped models and summarizes the few field data on wood induced hydraulics in streams and rivers. The focus is on the flow pattern and other effects of importance for instream ecology. The principal cross-flow field of a singular log perpendicular to flow is determined by the Reynolds number related to the log’s diameter. For the range of Reynolds numbers of logs and branches in streams and rivers (1 ‧ 10² to 1 ‧ 10⁶), the cross flow pattern is symmetrical, vortex streets shed, and a wake with reduced mean velocity develops behind the log. In the vertical confined flow of streams and rivers, the hydraulic effects depend on the blockage caused by the log, its distances to the water surface, and its distance to the streambed. The blockage determines the resistance to flow, the upstream afflux, the local flow acceleration, and the intensity of flow deflection. For logs within distances of 2 diameters to the water surface, the relative submergence and the Froude Number determine the highly variable local cross-flow field. For logs near the streambed, the form and roughness of the bed and the size of the gap to the bed control the hydraulics. Submerged jet-like flows, which cause local scour, are reported, but detailed information on the hydraulics of logs close to a natural streambed is missing. For logs in close contact to or partly embedded into the bed, the principal flow pattern of recirculating vortices attached to the bed develop in front and behind the logs. The extent of these vortices and the extent of the wake behind the logs appear to be larger in sand-bed streams than in flumes with smooth and level beds. Complex dense wooden objects and wood accumulations are comparable to solid structures. Their flow field is determined by the size of the bluff surfaces and the shedding from edges obtuse to flow. Wood spread out at the streambed causes skin roughness, and models based on technical roughness approximate the resulting near-bed flow regime. The general validity of most findings in streams and rivers is still vague since they are supported by only few data. Further flow data from the field and from flume experiments that simulate the complexity of the natural environment are needed.

The Ecology and Management of Wood in World Rivers

<em>Abstract.</em>—Wood in rivers, or wood deposited from fluvial processes, provides unique habitat for terrestrial and aquatic wildlife species. Many wildlife species utilize riparian areas for some portion of their life history primarily due to the universal need for water, the presence of unique plant assemblages, and the diversity of microhabitats produced by the dynamics of river systems. Wood in rivers provides four primary functions for aquatic and terrestrial wildlife species: habitat structure, shelter, patchiness of habitat, and increased food resources. Abundance and diversity of wildlife species are enhanced by wood in rivers, and they, in turn, shape and maintain aquatic and riparian habitats. Though there is a clear link between wood in rivers and riparian wildlife communities, knowledge about their interactions and interdependence is sparse.

The Ecology and Management of Wood in World Rivers

<em>Abstract.</em>—A large river, in relation to wood dynamics, has a width several times greater than the height of the trees in its riparian area. Large rivers undergo particular physical and biological processes related to wood that vary according to their condition (pristine or managed) and their locations in the landscape (upland area or downstream, tropical or temperate climates).

The Ecology and Management of Wood in World Rivers

<em>Abstract.</em>—Many catchments across the world have been highly modified by human activities, including agriculture, urban development, and other land uses, that often result in a complex landscape mosaic. We define developed catchments as those dominated by activities such as agriculture or urban development, irrespective of the extent and type of riparian zone present. Far fewer papers address large wood in rivers and streams within developed catchments compared with those in more natural situations, despite the fact that residential development and agricultural activities are so pervasive worldwide. The literature highlights a clear reduction of the abundance of large wood in agricultural and urban streams and rivers, although standing stocks are highly variable depending on local conditions. As a result of its scarcity, large wood seems to play a less important physical role than in forested ones. Nevertheless, large wood still plays an important role in developed streams and rivers by providing critical habitats for invertebrates and serving as the only retention structure remaining in some channels. A lower diversity of invertebrates and/or fishes and the loss of important functions, such as retention capacity, are reported for developed rivers, compared to those in forested regions. The geomorphic role of the wood remaining in these developed systems appears to be mixed— some studies report no such role, while others report an association of wood with pools. Gaps are evident in two areas: (1) many papers fail to adequately describe the landscape in which study streams and rivers are embedded, making it impossible to discern the dominant land use or, in some cases, even the nature and extent of the riparian vegetation; and (2) studies of ecosystem properties of streams and rivers in developed landscapes are rare. We suggest that more research should be undertaken in developed systems and that addressing the role of large wood is an important component of such studies.