Large wood and fluvial processes

Abstract

1. Large wood forms an important component of woodland river ecosystems. The relationship between large wood and the physical characteristics of river systems varies greatly with changes in the tree species of the marginal woodland, the climatic and hydrological regime, the fluvial geomorphological setting and the river and woodland management context. 2. Research on large wood and fluvial processes over the last 25 years has focussed on three main themes: the effects of wood on flow hydraulics; on the transfer of mineral and organic sediment; and on the geomorphology of river channels. 3. Analogies between wood and mineral sediment transfer processes (supply, mobility and river characteristics that affect retention) are found useful as a framework for synthesising current knowledge on large wood in rivers. 4. An important property of wood is its size when scaled to the size of the river channel. ′Small′ channels are defined as those whose width is less than the majority of wood pieces (e.g. width < median wood piece length). `Medium' channels have widths greater than the size of most wood pieces (e.g. width < upper quartile wood piece length), and `Large' channels are wider than the length of all of the wood pieces delivered to them. 5. A conceptual framework defined here for evaluating the storage and dynamics of wood in rivers ranks the relative importance of hydrological characteristics (flow regime, sediment transport regime), wood characteristics (piece size, buoyancy, morphological complexity) and geomorphological characteristics (channel width, geomorphological style) in `Small', `Medium' and `Large' rivers. 6. Wood pieces are large in comparison with river size in `small' rivers, therefore they tend to remain close to where they are delivered to the river and provide important structures in the stream, controlling rather than responding to the hydrological and sediment transfer characteristics of the river. 7. For `Medium' rivers, the combination of wood length and form becomes critical to the stability of wood within the channel. Wood accumulations form as a result of smaller or more mobile wood pieces accumulating behind key pieces. Wood transport is governed mainly by the flow regime and the buoyancy of the wood. Even quite large wood pieces may require partial burial to give them stability, so enhancing the importance of the sediment transport regime. 8. Wood dynamics in `Large' rivers vary with the geometry of the channel (slope and channel pattern), which controls the delivery, mobility and breakage of wood, and also the characteristics of the riparian zone, from where the greatest volume of wood is introduced. Wood retention depends on the channel pattern and the distribution of flow velocity. A large amount is stored at the channel margins. The greater the contact between the active channel and the forested floodplain and islands, the greater the quantity of wood that is stored.

Buffered, lagged, or cooled? Disentangling hyporheic influences on temperature cycles in stream channels

“…In alluvial rivers, large wood provides roughness elements that deflect flows, scour depressions, create gravel bars, facilitate channel braiding, [Abbe and Montgomery, 1996;Gurnell et al, 2002], and encourage channel avulsions that lead to the formation of anabranched channel patterns [Nanson and Knighton, 1996]. Associated complex bed topography and channel patterns enhance hydraulic gradients within the hyporheic zone and substantially increase hyporheic exchange [Dent et al, 2001;Cardenas et al, 2004;Lautz et al, 2006;Wondzell, 2006] while creating features such as spring channels that express resulting surface water variation in diel temperature cycles.…”

Factors influencing wood mobilization in streams

“…The most frequently cited predictor of mobilization that has emerged from these studies is the ratio of the length of a wood piece to the bankfull width of the stream channel [Gurnell et al, 2002;Hassan et al, 2005]. This length ratio is related to the probability that the piece of wood becomes braced against stream banks, rocks, or riparian trees before traveling an appreciable distance.…”

Material leñoso de gran tamaño en dos cuencas de la Cordillera de la Costa de Chile con diferente historia de uso del suelo

“…El volumen encontrado en Vuelta de Zorra (109 m 3 ha -1 ) se puede comparar con los encontrados en el río El Toro ubicado en la zona andina de la región de La Araucanía (117 m 3 ha -1 ; Andreoli et al 2008) y con el rango entre 100 y 200 m 3 ha -1 en bosques maduros no manejados de latifoliadas reportado por Gurnell (2003). Así mismo, este volumen es inferior a los 700 m 3 ha -1 encontrados en Tres Arroyos en la cordillera andina de la región de La Araucanía (Andreoli et al 2007), pero supera a lo informado en canales de segundo-tercer orden en cuencas con bosques manejados en las montañas italianas (30-70 m 3 ha -1 ; Comiti et al 2006), en cursos de agua de segundo-cuarto orden en los Highlands del Reino Unido (44-88 m 3 ha -1 ; Gurnell et al 2002), en canales relativamente inalterados de segundo-cuarto orden en Europa Central (20-85 m 3 ha -1 ; Kaczka 2003) y en un cauce de tercer orden de la Penínsu-la Ibérica (0,4-4,8 m 3 ha -1 ; Díez et al 2001). Los 56 m 3 ha -1 de material leñoso registrado en Pichún corresponden al menor volumen de material leñoso encontrado en sistemas lóticos chilenos.…”

“…La madera bien sea como elemento inundables adyacentes (Lenzi et al 2006). Numerosos es-individual o formando acumulaciones, crean condiciones tudios demuestran la importancia que tiene la presencia de que favorecen la sedimentación de partículas transportamaterial leñoso tanto en la morfología como en la ecología das por el agua, y en algunas cuencas de montaña, estos del sistema fluvial (Díez et al 2001, Gurnell et al 2002. sitios pueden almacenar de 10 a 15 veces el volumen del…”