The Natural Wood Regime in Rivers

Abstract

The natural wood regime forms the third leg of a tripod of physical processes that supports river science and management, along with the natural flow and sediment regimes. The wood regime consists of wood recruitment, transport, and storage in river corridors. Each of these components can be characterized in terms of magnitude, frequency, rate, timing, duration, and mode. We distinguish the natural wood regime, which occurs where human activities do not significantly alter the wood regime, and a target wood regime when management emphasizes wood recruitment, transport, and storage that balance desired geomorphic and ecological characteristics with mitigation of wood-related hazards. Wood regimes vary across space and through time, but can be inferred and quantified via direct measurements, reference sites, historical information, and numerical modeling. Classifying wood regimes with respect to wood process domains and quantifying the wood budget are valuable tools for assessing and managing rivers. Classic geomorphic conceptualizations of rivers focus exclusively on interactions between water and sediment (e.g., Lane’s balance, Lane 1955). Although water has sometimes been accorded dominance as a driving force on river process and form, the importance of sediment supply is also widely recognized. Boundary resistance to erosion is a fundamental influence on river process and form, and in this context the role of riparian vegetation is now well acknowledged, especially for low energy rivers (Gurnell et al. 2012, Gurnell 2014, Corenblit et al. 2015). Analogously, the effect of upland vegetation on sediment inputs to rivers is traditionally recognized for its role in limiting surface erosion and hillslope mass movement (e.g., Schumm 1968). The fundamental influence of vegetation as a geomorphic agent and as a source of wood to rivers is much less widely recognized in foundational literature, likely because of the long history of wood removal from river corridors by humans (Triska 1984, Montgomery et al. 2003, Wohl 2014). This last point is worth emphasizing: historical descriptions of forested regions throughout the temperate latitudes indicate that orders of magnitude more wood were present in most forested river corridors prior to widespread 2 deforestation and wood removal from river corridors for navigation and flood mitigation (Sedell and Froggatt 1984). In the context of this increasing knowledge of flow, sediment, and vegetation interactions, long-held arguments for the importance of a natural flow regime are based on the understanding that the geomorphic and ecological integrity of a river depend on its natural dynamic character. The original conceptualization of this dynamic character emphasized the importance of variations in fluxes of water through time (Poff et al. 1997). The conceptualization of a natural sediment regime broadened the consideration of a river’s dynamic character to reflect the importance of water and sediment interactions and sediment fluxes (Wohl et al. 2015). These two conceptual models recognize that centuries of human activities have created diverse changes in rivers, including alteration of natural flow and sediment regimes. These alterations have resulted in extensive ecological degradation and loss of biodiversity. Human activities on land and along rivers have also extensively changed and reduced important functions that include wood characteristics in river corridors. Alterations in the wood regime, however, are rarely recognized compared to the attention given to altered water and sediment regimes. Here, we argue that understanding the natural wood regime forms the third leg of a tripod supporting the physical processes underlying river science and management, along with the natural flow and sediment regimes. We define the wood regime in terms of the magnitude, frequency, rate, timing, duration, and mode of wood recruitment, transport, and storage. Large wood traditionally refers to downed, dead pieces greater than 10 cm in diameter and 1 m in length. Aggregates of smaller wood pieces (Culp et al. 1996, Galia et al. 2018) and living wood within the river corridor (Gurnell and Petts 2002, Gurnell et al. 2005, Opperman et al. 2008) also create important physical and ecological effects in river corridors. As a fundamental component of trees, wood contributes to the overall role of vegetation in driving forested river corridor form and function (Maser and Sedell 1994). Here, the river corridor includes fluvially influenced portions of a valley floor, such as the active channel(s), the floodplain and low terraces, the riparian zone, and the hyporheic zone. Explicit focus on river corridors, rather than channels, recognizes the vital importance of interactions between different portions of the valley bottom in the context of fluxes of water, sediment, and wood at networkto reach-scales (Hynes 1975). We consider a river corridor’s wood regime to include all sizes and types of wood. A rapidly growing literature documents the beneficial effects of wood on the geomorphology and ecology of rivers (Figure 1, Supplemental Table 1). Wood affects channel and floodplain ecological function via controls on riparian plant community development and structure, aquatic habitat, dynamics of particulate organic matter storage and processing, and the structure and production of biological communities. Wood influences longitudinal, lateral, and vertical fluxes of water, solutes, and mineral sediment – connectivity – within river corridors. Wood also changes channel and floodplain form both when the wood pieces are mobile and when they are stored. Failure to adequately consider these effects distorts our understanding of river process and form. On the other hand, wood transport can create flood hazards associated with wood accumulation at structures such as bridges, where jams can create substantial bed scour and flooding. Because of hazards and other constraints, a natural 3 wood regime may no longer be feasible in rivers with high flood risk. In these circumstances, a more pragmatic target wood regime should be identified and pursued to create at least some of the positive effects of wood in river ecosystems. Analogous to natural and altered water and sediment regimes, we draw a distinction in this paper between the natural wood regime and a target wood regime. A natural wood regime occurs where past and present human activities do not significantly alter the components of the wood regime. In many historically forested river corridors, however, human alterations of the mechanisms and magnitudes of the wood regime have been so sustained and intensive that it is no longer feasible to infer or restore a fully natural wood regime. Management can then be directed toward a target wood regime in which wood recruitment, transport, and storage balance desired geomorphic and ecological characteristics within the current landscape constraints and with mitigation of wood-related hazards. Our objectives in this paper are to (i) define and characterize the wood regime and (ii) provide perspectives on how to characterize and manage for natural and target wood regimes to increase geomorphic and ecological integrity of river corridors.