18
runoff or tributary inputs. Finally, sediment can be remobilized within the river corridor from the stream bed, banks, in-channel bars, and floodplain. Detailed sediment budgets indicate that in some rivers, such as the Amazon, more sediment
enters the active channel from bank erosion and lateral channel movement across
the floodplain than is transported from upstream sources (Dunne et al. 1998).
Diverse river drainage basins also receive the majority of upland sediment inputs
from a relatively small portion of the total watershed (Meade 2007; Meade and
Moody 2010). Understanding the sources of sediment and the processes by which
sediment enters and moves within the river corridor becomes particularly important
when conditions of sediment deficit or excess create changes in channel form and
stability or in nutrient supply.
Processes such as landslides and debris flows can recruit large wood to the river
corridor from adjacent uplands (Abbe and Montgomery 2003; May and Gresswell
2003). Large wood can also be recruited from floodplain forests via individual tree
fall or mass mortality during blowdowns, insect infestations, or wildfires. Bank erosion also topples floodplain trees, and river erosion exhumes wood buried in floodplain sediment (Benda and Sias 2003). Whatever the source, large wood in the
channel increases resistance to flow (Curran and Wohl 2003; Hygelund and Manga
2003) and creates obstructions that enhance hyporheic exchange (Hester and Doyle
2008; Sawyer et al. 2011). Flow deflection around the wood drives local scour of the
bed and banks, but also deposition of finer sediment and particulate organic matter
in areas of lower velocity associated with the wood (Battin et al. 2008; Beckman
and Wohl 2014). Particulate organic matter (>0.45 μm in size) is composed of materials such as fragments of leaves and twigs. Scour and deposition around large wood
increase the abundance and diversity of aquatic habitat for a variety of organisms
(Richmond and Fausch 1995; Johnson et al. 2003). Large wood can affect the type
and dimensions of bedforms (MacFarlane and Wohl 2003) and either increase the
spatial heterogeneity of bed substrate by creating patches of differently sized sediment (Buffington and Montgomery 1999), or allow alluvial substrate to accumulate
in what would otherwise be a bedrock channel (Massong and Montgomery 2000).
Logjams, in particular, can facilitate formation of anabranching channels (O’Connor
et al. 2003; Wohl 2011; Collins et al. 2012) and enhance channel-floodplain connectivity and patterns of overbank erosion and deposition (Jeffries et al. 2003).
Although most contemporary studies of wood in rivers focus on small to mediumsized rivers, even the largest rivers historically had substantial quantities of wood
(Triska 1984; Montgomery et al. 2003; Wohl 2014a). Large wood continues to
influence some large rivers with less alteration of land cover in the drainage basin
(e.g., Martín-Vide et al. 2014; Boivin et al. 2015; Kramer et al. 2017).
Floodplain large wood also influences flow resistance and patterns of erosion
and deposition during overbank flows, as well as providing vital habitat for diverse
types of plants and animals during periods of floodplain inundation and emergence.
Case studies illustrating the role of floodplain large wood include the survival of
riparian seedlings along the Sabie River in semiarid South Africa (Pettit and
Naiman 2006); macroinvertebrate habitat in subtropical coastal plain rivers of
2 Rivers as Ecosystems
runoff or tributary inputs. Finally, sediment can be remobilized within the river corridor from the stream bed, banks, in-channel bars, and floodplain. Detailed sediment budgets indicate that in some rivers, such as the Amazon, more sediment
enters the active channel from bank erosion and lateral channel movement across
the floodplain than is transported from upstream sources (Dunne et al. 1998).
Diverse river drainage basins also receive the majority of upland sediment inputs
from a relatively small portion of the total watershed (Meade 2007; Meade and
Moody 2010). Understanding the sources of sediment and the processes by which
sediment enters and moves within the river corridor becomes particularly important
when conditions of sediment deficit or excess create changes in channel form and
stability or in nutrient supply.
Processes such as landslides and debris flows can recruit large wood to the river
corridor from adjacent uplands (Abbe and Montgomery 2003; May and Gresswell
2003). Large wood can also be recruited from floodplain forests via individual tree
fall or mass mortality during blowdowns, insect infestations, or wildfires. Bank erosion also topples floodplain trees, and river erosion exhumes wood buried in floodplain sediment (Benda and Sias 2003). Whatever the source, large wood in the
channel increases resistance to flow (Curran and Wohl 2003; Hygelund and Manga
2003) and creates obstructions that enhance hyporheic exchange (Hester and Doyle
2008; Sawyer et al. 2011). Flow deflection around the wood drives local scour of the
bed and banks, but also deposition of finer sediment and particulate organic matter
in areas of lower velocity associated with the wood (Battin et al. 2008; Beckman
and Wohl 2014). Particulate organic matter (>0.45 μm in size) is composed of materials such as fragments of leaves and twigs. Scour and deposition around large wood
increase the abundance and diversity of aquatic habitat for a variety of organisms
(Richmond and Fausch 1995; Johnson et al. 2003). Large wood can affect the type
and dimensions of bedforms (MacFarlane and Wohl 2003) and either increase the
spatial heterogeneity of bed substrate by creating patches of differently sized sediment (Buffington and Montgomery 1999), or allow alluvial substrate to accumulate
in what would otherwise be a bedrock channel (Massong and Montgomery 2000).
Logjams, in particular, can facilitate formation of anabranching channels (O’Connor
et al. 2003; Wohl 2011; Collins et al. 2012) and enhance channel-floodplain connectivity and patterns of overbank erosion and deposition (Jeffries et al. 2003).
Although most contemporary studies of wood in rivers focus on small to mediumsized rivers, even the largest rivers historically had substantial quantities of wood
(Triska 1984; Montgomery et al. 2003; Wohl 2014a). Large wood continues to
influence some large rivers with less alteration of land cover in the drainage basin
(e.g., Martín-Vide et al. 2014; Boivin et al. 2015; Kramer et al. 2017).
Floodplain large wood also influences flow resistance and patterns of erosion
and deposition during overbank flows, as well as providing vital habitat for diverse
types of plants and animals during periods of floodplain inundation and emergence.
Case studies illustrating the role of floodplain large wood include the survival of
riparian seedlings along the Sabie River in semiarid South Africa (Pettit and
Naiman 2006); macroinvertebrate habitat in subtropical coastal plain rivers of
2 Rivers as Ecosystems