16
(Stanford and Ward 1988). The length and travel time of flow paths within the
hyporheic zone vary from a few meters and minutes to hundreds of meters and
many hours across the river corridor and downstream, as well as through time in
response to fluctuations in river discharge (Gooseff 2010). Hyporheic flow can constitute less than one percent of river discharge in steep, small channels with limited
alluvium (Wondzell and Swanson 1996), and 15% or more of surface discharge in
larger, lowland alluvial rivers (Laenen and Risley 1997).
Explicitly recognizing different components within a river corridor is inherent in
perceiving and managing rivers as ecosystems. If a river is conceptualized simply as
a feature for downstream conveyance, then management focuses on the active channel and is more likely to result in severing the active channel from the adjacent
floodplain and underlying hyporheic zone. One result of such artificial compartmentalization is that the exchanges of energy, materials, and organisms among the
channel, floodplain, and hyporheic zone are likely to be disrupted in a manner that
impairs river health.
2.2 Controls on Physical Form and Process in River
Corridors
Water and sediment inputs are the primary drivers of physical form and process in
all rivers (Fig. 2.4). In forested river corridors, large wood (> 10 cm diameter and
1 m length) historically formed an equally important component. However, centuries of large wood removal in many river networks have reduced the influence of
wood and allowed even river scientists to forget how strong an influence wood once
exerted on channel and floodplain process and form (Montgomery et al. 2003;
Wohl 2014a).
Water flowing downstream converts potential energy to kinetic energy. A river
expends flow energy in three ways: overcoming frictional resistance from the channel boundaries and within the flow; transporting sediment; and eroding the channel
boundaries. The amount of energy available at any point in space and time reflects
the volume of water flowing downstream and the valley and channel geometry,
which influence downstream gradient and frictional resistance. The work exerted by
the flow reflects the balance between available energy and the erosional resistance
of the channel substrate. Substrate erosional resistance results from substrate composition (e.g., sand versus bedrock) and additional resistance created by living
aquatic and riparian vegetation and dead vegetation in the form of instream and
floodplain wood.
Understanding the manner in which flow energy is used becomes important in
the context of river corridor stability. If flow energy increases substantially because
of greater discharge, such as during a flood, available energy can exceed the erosional resistance of the channel boundaries, resulting in erosion of the bed or channel widening. Conversely, if flow energy is insufficient to perform the work of
2 Rivers as Ecosystems
(Stanford and Ward 1988). The length and travel time of flow paths within the
hyporheic zone vary from a few meters and minutes to hundreds of meters and
many hours across the river corridor and downstream, as well as through time in
response to fluctuations in river discharge (Gooseff 2010). Hyporheic flow can constitute less than one percent of river discharge in steep, small channels with limited
alluvium (Wondzell and Swanson 1996), and 15% or more of surface discharge in
larger, lowland alluvial rivers (Laenen and Risley 1997).
Explicitly recognizing different components within a river corridor is inherent in
perceiving and managing rivers as ecosystems. If a river is conceptualized simply as
a feature for downstream conveyance, then management focuses on the active channel and is more likely to result in severing the active channel from the adjacent
floodplain and underlying hyporheic zone. One result of such artificial compartmentalization is that the exchanges of energy, materials, and organisms among the
channel, floodplain, and hyporheic zone are likely to be disrupted in a manner that
impairs river health.
2.2 Controls on Physical Form and Process in River
Corridors
Water and sediment inputs are the primary drivers of physical form and process in
all rivers (Fig. 2.4). In forested river corridors, large wood (> 10 cm diameter and
1 m length) historically formed an equally important component. However, centuries of large wood removal in many river networks have reduced the influence of
wood and allowed even river scientists to forget how strong an influence wood once
exerted on channel and floodplain process and form (Montgomery et al. 2003;
Wohl 2014a).
Water flowing downstream converts potential energy to kinetic energy. A river
expends flow energy in three ways: overcoming frictional resistance from the channel boundaries and within the flow; transporting sediment; and eroding the channel
boundaries. The amount of energy available at any point in space and time reflects
the volume of water flowing downstream and the valley and channel geometry,
which influence downstream gradient and frictional resistance. The work exerted by
the flow reflects the balance between available energy and the erosional resistance
of the channel substrate. Substrate erosional resistance results from substrate composition (e.g., sand versus bedrock) and additional resistance created by living
aquatic and riparian vegetation and dead vegetation in the form of instream and
floodplain wood.
Understanding the manner in which flow energy is used becomes important in
the context of river corridor stability. If flow energy increases substantially because
of greater discharge, such as during a flood, available energy can exceed the erosional resistance of the channel boundaries, resulting in erosion of the bed or channel widening. Conversely, if flow energy is insufficient to perform the work of
2 Rivers as Ecosystems