15
ordinary high water mark can be very difficult in regions with extremely variable
flow, such as drylands or tropical regions, because a recent extraordinary flood may
have created erosional and depositional features that obscure the indicators left by
ordinary flows (Lichvar and Wakeley 2004).
A floodplain is a relatively flat sedimentary surface adjacent to the active channel
that is built by contemporary river processes and inundated frequently (Nanson and
Croke 1992; Dunne and Aalto 2013). The lateral extent of a floodplain can be
defined in a regulatory context based on the area inundated by a flow with a specific
return interval: this leads to definition of a 10-year floodplain, for example, or a 100year floodplain. River scientists are more likely to describe a floodplain as the surface that is flooded at least once every two years. Rivers with highly variable flow,
however, such as those in drylands, can have flat surfaces adjacent to the channel
and composed of contemporary river sediments that are flooded only every few
decades (Williams 1978a).
In this book, floodplain includes both the floodplain and the riparian zone. The
riparian zone is the interface between terrestrial and aquatic ecosystems (Naiman
et al. 2005) and, like the floodplain, can be difficult to delineate on the ground. The
US Army Corps of Engineers, which oversees regulatory delineations for freshwaters in the United States, defines riparian areas as “lands adjacent to streams ….
transitional between terrestrial and aquatic ecosystems, through which surface and
subsurface hydrology connects riverine, lacustrine, estuarine, and marine waters
with their adjacent wetlands, non-wetland waters, or uplands” (USACE 2012).
Riparian zones can include features as diverse as depressions that create floodplain
wetlands and higher-elevation natural levees. The lateral extent of the riparian zone
can also be defined as the surface extent of frequent flooding and the subsurface
extent of mixing of river, hyporheic, and ground waters (Wohl 2014b), creating
substantial overlap with the definition of floodplains. Riparian zone is primarily an
ecological concept and floodplain is primarily a geomorphic and hydrologic concept. In addition, riparian zones can occur in portions of the river network that do
not have floodplains, such as steep sided, narrow canyons.
The floodplain can also include depositional features associated with repeated
fluxes of sediment into the main stem river corridor, such as alluvial fans created by
tributary streams (Stock 2013) or debris cones created by rockfall or landslides
(Harvey 2012). These features can have spatially and temporally complicated interactions with the active channel and floodplain. Fans and cones can expand into the
river corridor during periods of large sediment influxes, for example, and then
undergo periods of contraction when the active channel erodes the toe of the alluvial
fan or debris cone. Where present, fans and cones tend to create at least temporary
lateral disconnectivity of water, sediment, and nutrient fluxes into the river corridor,
as well as influencing the shape and longitudinal connectivity of the river corridor
(Harvey 2012; Fryirs et al. 2007).
The hyporheic zone consists of unconfined, near-stream aquifers where river
water is present because flow paths originate and terminate at the channel. This zone
can extend several meters below large alluvial rivers and as far as 2 km laterally
from the active channel in river corridors with broad floodplains underlain by gravel
2.1 Physical Components of the River Corridor
ordinary high water mark can be very difficult in regions with extremely variable
flow, such as drylands or tropical regions, because a recent extraordinary flood may
have created erosional and depositional features that obscure the indicators left by
ordinary flows (Lichvar and Wakeley 2004).
A floodplain is a relatively flat sedimentary surface adjacent to the active channel
that is built by contemporary river processes and inundated frequently (Nanson and
Croke 1992; Dunne and Aalto 2013). The lateral extent of a floodplain can be
defined in a regulatory context based on the area inundated by a flow with a specific
return interval: this leads to definition of a 10-year floodplain, for example, or a 100year floodplain. River scientists are more likely to describe a floodplain as the surface that is flooded at least once every two years. Rivers with highly variable flow,
however, such as those in drylands, can have flat surfaces adjacent to the channel
and composed of contemporary river sediments that are flooded only every few
decades (Williams 1978a).
In this book, floodplain includes both the floodplain and the riparian zone. The
riparian zone is the interface between terrestrial and aquatic ecosystems (Naiman
et al. 2005) and, like the floodplain, can be difficult to delineate on the ground. The
US Army Corps of Engineers, which oversees regulatory delineations for freshwaters in the United States, defines riparian areas as “lands adjacent to streams ….
transitional between terrestrial and aquatic ecosystems, through which surface and
subsurface hydrology connects riverine, lacustrine, estuarine, and marine waters
with their adjacent wetlands, non-wetland waters, or uplands” (USACE 2012).
Riparian zones can include features as diverse as depressions that create floodplain
wetlands and higher-elevation natural levees. The lateral extent of the riparian zone
can also be defined as the surface extent of frequent flooding and the subsurface
extent of mixing of river, hyporheic, and ground waters (Wohl 2014b), creating
substantial overlap with the definition of floodplains. Riparian zone is primarily an
ecological concept and floodplain is primarily a geomorphic and hydrologic concept. In addition, riparian zones can occur in portions of the river network that do
not have floodplains, such as steep sided, narrow canyons.
The floodplain can also include depositional features associated with repeated
fluxes of sediment into the main stem river corridor, such as alluvial fans created by
tributary streams (Stock 2013) or debris cones created by rockfall or landslides
(Harvey 2012). These features can have spatially and temporally complicated interactions with the active channel and floodplain. Fans and cones can expand into the
river corridor during periods of large sediment influxes, for example, and then
undergo periods of contraction when the active channel erodes the toe of the alluvial
fan or debris cone. Where present, fans and cones tend to create at least temporary
lateral disconnectivity of water, sediment, and nutrient fluxes into the river corridor,
as well as influencing the shape and longitudinal connectivity of the river corridor
(Harvey 2012; Fryirs et al. 2007).
The hyporheic zone consists of unconfined, near-stream aquifers where river
water is present because flow paths originate and terminate at the channel. This zone
can extend several meters below large alluvial rivers and as far as 2 km laterally
from the active channel in river corridors with broad floodplains underlain by gravel
2.1 Physical Components of the River Corridor