The characteristics of woody debris and sediment distribution in headwater streams, southeastern Alaska

Abstract

Large woody debris (LWD), fine woody debris (FWD), fine organic debris (FOD), and sediment deposition were measured in 15 steep headwater streams with five management and disturbance regimes. Clear-cut channels logged in 1995 contained large accumulations of logging residue that initially provided sites for sediment storage. Half of the LWD in clear-cut channels was recruited during and immediately after logging. Woody debris from logging activities remains in young growth conifer channels 37 years after logging. Numbers of LWD in clear-cut and young conifer channels were significantly higher than in old-growth channels, although numbers of FWD pieces were not significantly different because of higher recruitment from old-growth stands. Channels that experienced recent (1979 and (or) 1993) and earlier (1961 and (or) 1979) scour and runout of landslides and debris flows contained less LWD and FWD, although large volumes of LWD and FWD were found in deposition zones. The volumes of sediment stored in young alder and recent landslide channels were higher than in the other channels. Because of the recruitment of LWD and FWD from young alder stands, the ratio of sediment stored behind woody debris to total sediment volume was higher in young alder channels compared with recent landslide channels. Numbers of LWD and FWD pieces in all streams were significantly correlated with the volumes of sediment stored behind woody debris. Timber harvesting and soil mass movement influence the recruitment, distribution, and accumulation of woody debris in headwater streams; this modifies sediment storage and transport in headwater channels.Résumé : Les débris ligneux grossiers (DLG), les débris ligneux fins (DLF), les débris organiques fins (DOF) et le dé-pôt de sédiments ont été mesurés dans 15 têtes de rivière escarpées avec cinq régimes d'aménagement et de perturbation. Des vallées coupées à blanc en 1995 contenaient d'importantes accumulations de résidus de coupe qui ont initialement fourni des sites pour la rétention des sédiments. La moitié des DLG dans les vallées coupées à blanc ont été accumulés pendant et immédiatement après la coupe. Les débris ligneux provenant des activités de récolte persistent dans les vallées avec une régénération de conifères 37 ans après la coupe. Le nombre de DLG dans les vallées coupées à blanc et les vallées avec de jeunes conifères est significativement plus élevé que dans les vallées de vieille forêt, quoique que le nombre de morceaux de DLF n'ait pas été significativement différent à cause du recrutement plus élevé dans les peuplements de vieille forêt. Les vallées qui ont subi de récents (1979 et (ou) 1993) et plus vieux (1961 et (ou) 1979) décapages et écoulements suite à des glissements de terrain et des mouvements de débris contenaient moins de DLG et DLF, bien que d'importants volumes de DLG et DLF aient été retrouvés dans les zones de déposi-tion. Le volume de sédiments retenus dans les vallées couvertes de jeunes aulnes et celles qui avaient récemment subi des glisseme...

Understanding Processes and Downstream Linkages of Headwater Systems

“…Sediment and woody debris are routed as channelized debris flows and deposited in the downstream reaches of headwater systems (Benda and Cundy 1990). Exposed bedrock and less woody debris typify scour and runout zones (Gomi et al 2001). In contrast, massive piles of woody debris and sediment are found in deposition zones of debris flows (Hogan et al 1995).…”

“…Adjustment of channel morphology after landslides and debris flows largely depends on sediment and woody debris inputs. The regeneration of riparian stands in scour and deposition zones of debris flows begins to restore the recruitment of woody debris 20 to 50 years after mass movement in headwater streams (Gomi et al 2001).…”

“…Because substrate materials are not well sorted, interlocking boulders and cobbles modify the stability of channels, forming channel steps and creating sites for sediment storage (Zimmerman and Church 2001). Woody debris pieces also store sediment and modify channel roughness, and owing to the narrow channel width, relatively small woody debris pieces and jams have similar functions in headwater channels (Gomi et al 2001). Therefore, headwater streams may have more woody debris dams than larger watershed systems because smaller woody debris can form these dams (figure 3c).…”

Organización y ajuste de sedimentos en un tramo torrencial de la cabecera del río Ijuez, Pirineo central español

“…Liébault et al (1999) noted that a high level of spatial variability is typical of small mountain streams, because of coarse sediment inputs from hillslopes, bank erosion and the presence of large organic debris in the channels. Large woody debris can retain sediment for years or decades (Bovis et al, 1998), and become transitory sediment sinks (Nakamura and Swanson, 1993;Gomi et al, 2001). During floods these structures can deliver large volumes of sediment, reinforcing the episodic nature of sediment transport in mountain rivers (Church, 2010).…”

Implementation of the fire and fire surrogate study— a national research effort to evaluate the consequences of fuel reduction treatments

“…Most landslides generated on steep deglaciated slopes of southeast Alaska enter streams and deposit at the base of wide U-shaped valleys on slopes between 5 and 20 degrees (Gomi et al 2001, Johnson et al 2000, Swanston and Marion 1991; slopes similar to those selected for ATC study sites. Thus, although landslide initiation may occur in steep, nonfish bearing reaches, the ultimate effects often occur in lower gradient, fish-bearing reaches.…”