The Ecology and Management of Wood in World Rivers

Dynamics of Wood in Large Rivers

Herve Piégay


Abstract.—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).

In this chapter, several topics are developed to illustrate how large wood plays a significant role in the functioning of large rivers: production and transfer of wood, interactions between wood and channel geometry (accumulation sites, trapping efficiency, effects of wood on channel forms), and interactions between wood and human activities and management (effects of wood on fish resources, damages associated with the wood, restoration and maintenance strategies).

Bank erosion, wind, beavers, floodplain forest clearing by overbank flow, meteorological events (snow, wind), and humans (dump areas, residuals of timber harvest) are the main vectors of wood introduction to large rivers. The amount of wood stored in the channel is usually low compared to what is introduced annually. Wood is mainly located at the edge of the floodplain (islands, concave banks, side channels), but differences exist in accumulation sites, according to the channel pattern and geometry. The effects of wood on island formation as well as its effects on meander cut-off have been observed but are variable from one river to another according to the size of the wood and the character of the floodplain.

Fish abundance and diversity are influenced by wood structures, even in rivers with low amounts of wood. Wood obstructions and their associated geomorphic facies provide rearing sites and habitats for aquatic invertebrates. Because the amount of wood in large rivers is usually low, it rarely increases flooding risks locally. Wood affects navigation where unobstructed channels are favored; wood transport has then a cost because it must be removed. When the wood in rivers is enhancing fish resources, there is a need to define a balance between the ecological benefit of the wood and the need for removing it for safety purposes. In the 1980s, research was focused on habitat questions at a local spatial scale, linking hydraulic and geomorphic conditions, and characters of fish species. A research priority is now necessary to understand wood mobility and its downstream impacts.