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Journal of Hydrology (NZ) 50 (1): 161-180 2011
© New Zealand Hydrological Society (2011)
Conceptualization of sediment flux in the
Tongariro catchment
Gary J. Brierley,1 Helen E. Reid1 and Stephen E. Coleman2
1
School of Environment, The University of Auckland, Auckland, New Zealand.
Corresponding author: [email protected]
2 Department of Civil and Environmental Engineering, The University of Auckland,
Auckland, New Zealand.
Abstract
To quantify sediment flux in river systems
requires not only measurements (or estimates)
of how much sediment moves through
channel cross-sections or reaches over a given
time period, but also consideration of how
representative those measurements are and
how long those rates of sediment movement
will be sustained. Geomorphic analysis
must accompany application of engineering
principles to develop this understanding,
explaining controls on sediment availability
for a particular system. Geomorphic controls
on sediment flux are discussed here in terms
of four principles: landscape setting, landscape
connectivity, reach sensitivity and sediment
organization. These principles are applied
in the Tongariro catchment. The landscape
setting of this catchment is fashioned
primarily by the volcanic history of the area,
with abundant sediment supply. Landscape
connectivity is limited, with many sediment
stores in upland areas disconnected from the
lower course of the river. Reach sensitivity
downstream of the gorge is limited by the
area of active channel inset within terraces.
Significant channel adjustments have oc­
curred in the braided reach beyond the terraceconfined reaches. This reach has acted as an
efficient trap for gravel-sized materials, such
that further downstream, the meandering
and multi-channeled delta reaches comprise
sand-sized materials. These latter reaches have
shown negligible channel adjustments over the
last 80 years. Collectively, these inter-related
controls determine variability in sediment
availability in the Tongariro catchment over
time, thereby exerting a dominant influence
upon sediment flux. Human disturbance is
concluded to have had a negligible impact on
sediment flux in this resilient system.
Key words
Landscape setting, landscape memory,
landscape connectivity, river sensitivity, bed
material organization, planform adjustment,
River Style
Introduction
Explaining variability in sediment flux is a
key aspect for evaluating river management
applications with respect to habitat
availability and viability, the sustainability of
resource extraction, and concerns for erosion,
sedimentation and flooding hazards (e.g.,
Sear, 1994; Sear et al., 1995). Understanding
of controls on natural variability in sediment
flux is important for determining how human
disturbance has affected river adjustments
(Richards, 2002). Approaches to analysis of
sediment flux vary markedly depending on
the purpose of the exercise (Reid and Dunne,
161
1996). Sediment movement can be measured
in the field with reasonable accuracy over
short periods (hours/days) and modelling
applications can be used to quantify longterm average rates of sediment flux under
assumed sets of conditions. Indeed, we
have good theoretical and experimental
(flume-based) understanding of hydraulic
controls on sediment flux in river channels.
However, analyses of the landscape controls
that determine sediment availability within
a specific catchment, and the efficiency of
sediment transfer to the channel itself, are
poorly established. Spatial and temporal
variability in sediment flux often confounds
our ability to generate reliable sediment
rating curves.
This paper outlines a conceptual
approach to analysing geomorphic controls
on movement of bedload materials at the
catchment scale. Factors that determine
sediment availability, the frequency with
which sediment stores are reworked, and how
readily stores are replenished are used to assess
how representative sediment movement is at
any given place over any given time interval.
The approach developed here is qualitative,
but future applications could quantify the
relative importance of the primary controls
upon sediment flux.
We first briefly summarize conventional
approaches to analysis of sediment flux
based upon cross-section-scale application
of engineering principles. The geomorphic
(landscape) approach to analysis of controls
upon sediment flux is then documented
(Fig. 1). This builds upon four components:
landscape setting (geologic and climatic
conditions), the inter-reach connectivity
of a landscape (how readily sediments are
conveyed from hillslopes to valley floors and
from reach to reach along the river), reachscale sensitivity to geomorphic adjustment, and
the organization of sediments within reaches
(their ease of mobility). These concepts are
then used to outline controls on sediment
162
flux within the Tongariro catchment. We then
assess the effects of human disturbance in
relation to ‘natural’ variability, and highlight
implications for the management of sediment
flux at the catchment scale.
Geomorphic aspects affecting
sediment flux
Consideration of scale
As indicated in many papers in this special
issue, en­gineer­ing analysis of sediment flux
focuses on sediment continuity through
erosion, transport, and deposition. This focus
is typically at channel cross-section scales, and
is scaled up to provide reach- and catchmentscale descriptions of sediment processes.
This approach is inadequate to describe flux
variations along reaches, as it focuses solely
upon sediment moving through a crosssection, and fails to consider factors such as
patches of armouring and bedrock that limit
sediment supply, and local erosion at subreach scales. More importantly, if uniform
upscaling is used, significant catchmentscale controls on sediment movement and
storage will be overlooked or not addressed.
Numerical models cannot capture the
inherent complexity of the natural world,
and unavoidable assumptions must be
made to simplify model behaviour (Merritt
et al., 2003). Equations and relations used
in sediment transport models are typically
one dimensional, and often reflect empirical
relationships derived elsewhere (e.g.,
Brasington and Richards, 2007). Lumped
simulation models are furthermore typically
framed in relation to average flow and
sediment conditions. It is thus unlikely that
the mechanistic, one-dimensional reasoning
behind conventional numerical modelling
can be applied to anything beyond small-scale
spatial and temporal processes. To compensate
for this, various non-fluid properties that
affect bedload transport must be incorporated
into these modelling applications (Lane
et al., 2008). These reframed applications
must consider geomorphic properties such
as landscape setting, landscape connectivity,
planform geometry, and bed material
organization.
A general picture of sediment flux through
a catchment
Rivers can only move the sediments that
are available to them. Sediment generation
is determined primarily by the landscape
setting, in particular tectonic controls and
the erodibility of the prevailing lithology
(Richards, 2002). These are interrelated,
in that the conditions that create high
availability of sediment via uplift also
create significant relief via incision, thereby
inducing high erosion rates via landscape
dissection. Uplifting, dissected terrains are
highly-connected landscapes, with effective
and efficient sediment delivery from hillslopes
to valley floors, and thence downstream
along steep and narrow confined valleys.
Rapid transfer of high volumes of material
promotes further incision. Over time, and
with distance downstream, valleys widen
and sediment stores become more recurrent
and permanent features. These bodies of
sediment are themselves prone to reworking,
with variable residence times. Channel
planform and channel geometry reflect the
way a channel adjusts and moves sediment on
valley floors. Resulting patterns of sediment
stores and sinks, and their material properties,
influence the ease and frequency with which
sediments are reworked and redistributed.
These geomorphic principles are brought
together in Figure 1 to provide a conceptual
overview of geomorphic controls on sediment
flux.
Landscape setting
Rivers must be viewed in their landscape
context. Geological controls determine
the relief, topography and erodibility of a
landscape. The balance of uplift and erosion
sets the boundary conditions for sediment
generation and reworking. Across New
Zealand, for example, there are profound
differences in river types and sediment flux
in landscapes such as the rapidly uplifting
weak rocks of the East Coast region of the
North Island, the volcanic landscapes of the
central/northern part of the North Island,
and the uplifting glaciated
landscapes of the Southern
Alps (cf., Adams, 1979;
Griffiths, 1979; 1982; Hicks
et al., 1996). Sediment
availability, and associated
patterns of sediment stores
and sinks, vary markedly in
these differing landscapes,
in terms of both the volume
and calibre of material.
Lithology influences the
hardness and size of break­
down products of these
materials. The relationship between relief (slope)
and valley width also deter­
Figure 1 – Cross-scalar relationships that determine sediment flux mines the accommodation
at the catchment scale
space for sediment storage.
163
These considerations determine patterns of
sediment source, transfer and accumulation
zones in any given catchment (Schumm,
1977; Brierley and Fryirs, 2005).
The climate regime and vegetation cover
affect the magnitude, frequency and ease
with which materials are eroded from hill­
slopes, transferred to valley floors, and
conveyed downstream. Regeneration of sedi­
ments upon hillslopes, determined largely
by tectonic controls on uplift and lithologic/
climatic controls upon weathering rates,
determines how readily sediment stores can
be replenished, making materials available to
be eroded and/or reworked. Some landscapes
are essentially stripped of readily-available
sediments. In these supply-limited land­scapes, sediments that do become available
are readily flushed through the system. In
contrast, transport-limited landscapes may
become choked with sediment, promoting
rapid and extensive aggradation on valley
floors. Residence times for sediment
reworking vary markedly for differing sedi­
ment storage units, whether on hillslopes
or along the valley floor (both channel and
floodplain compartments) (Brown, 1987;
Phillips, 2003). As climate conditions and
land use change over time, so too does the
nature and distribution of vegetation and
resulting patterns of sedi­ment flux.
Landscape setting is a primary influence
upon the sensitivity of landscapes to human
disturbance. For example, removal of forest
cover promoted extensive landslide and gully
mass movement activity, which induced rapid
accumulation of materials on valley floors
in the East Coast region of New Zealand
(Hicks et al., 2000; Page et al., 2007). Other
than in areas with historical alluvial mining
activities, such dramatic landscape responses
to human disturbance are uncommon
elsewhere in New Zealand. A significant
proportion of the large volume of sediment
liberated by human disturbance may be
stored downstream (the so-called sediment
164
delivery problem). These sediment stores
have been referred to as ‘legacy sediments’
(e.g., James, 2010). Wilkinson and McElroy
(2007: 140) contend that “(a)ccumulation
of post-settlement alluvium on higher-order
tributary channels and floodplains … is the
most important geomorphic process in terms
of the erosion and deposition of sediment that
is currently shaping the landscape of Earth.”
In some settings, human activities may result
in decreased rates of sediment movement
over time as sediment sources are depleted or
exhausted (e.g., Brooks and Brierley, 2004;
Fryirs and Brierley, 2001; Hudson, 2003).
The impact of dams upon sediment flux is
especially profound (Vörösmarty et al., 2003).
Hence, human disturbance may enhance or
suppress rates of sediment flux.
Contemporary human impacts upon
sediment flux need to be placed in the
context of the history (or memory) of any
given landscape (Phillips, 2007; Brierley
2010 a, b). For example, earthquakes or
volcanic eruptions may impart a persistent
long-term imprint upon the landscape,
generating and redistributing large volumes
of materials (e.g., Simon, 1992; Montgomery
et al., 1999). Many landscapes have an
imprint of past climatic events. For example,
the contemporary sediment load of many
rivers flowing from glaciated areas has been
greatly influenced by reworking of glaciallyderived sediments (paraglacial materials),
which create vast fan and terrace sequences
along some rivers (see Ballantyne, 2002;
Church and Slaymaker, 1989). Finally, past
human actions may affect the contemporary
behaviour of a system.
The River Styles framework (Brierley and
Fryirs, 2005) provides a geomorphic tool for
appraising the influence of landscape setting
upon catchment-scale patterns and linkages of
river types. This approach to river classification
uses reach-scale assemblages of geomorphic
units (channel and floodplain features that
reflect distinctive relationships between
process and form). Source, transfer and
accumulation zones are differentiated on the
basis of patterns of erosional and depositional
landforms in differing valley settings. Confined
valleys are predominantly erosional features
in source zones. Partly confined valleys have
discontinuous floodplain pockets in transfer
zones. Laterally unconfined valleys have an
array of sediment stores and sinks, typically
acting as transfer or accumulation zones.
The River Styles framework can be used to
appraise geomorphic controls on sediment
flux, relating the package of erosional and
depositional features in any given reach to
the pattern of river types in a catchment (and
associated implications for sediment transfer
from reach to reach).
The imprint of landscape connectivity between
reaches on sediment flux
The manner and extent to which sediments
are conveyed through a catchment reflects
the connectivity or coupling of that particular
system (Harvey, 2002; Hooke, 2003).
Connectivity must be maintained throughout
a system if inputs from headwater source
zones are to become outputs at the basin
mouth. Viewed in this way, the sediment
delivery ratio provides a measure of the
effectiveness of landscape connectivity. Forms
and strengths of landscape connectivity can
vary dramatically throughout a catchment
and over time (Brierley et al., 2006; Jain and
Tandon, 2010).
Two key components of landscape
connectivity are the transfer of sediments
from hillslopes to valley floors (a form of
lateral connectivity), and reach-to-reach
transfer of sediments along the valley floor
itself (longitudinal connectivity). In highly
coupled systems, water and sediment from
hillslopes are delivered directly to the channel
(Harvey, 2002; Fryirs et al., 2007). Moving
downstream, slope flattens and valley floors
widen, increasing accommodation space and
the capacity for sediment storage (Lane and
Richards, 1997). Various landforms may
impede sediment conveyance, either within
landscape compartments or within the
catchment as a whole. Some landforms buffer
sediment delivery from hillslopes to valley
floors, while other landforms act as barriers
that inhibit the efficiency of sediment transfer
along valley floors (Fryirs et al., 2007).
In some settings, slopes and channels are
decoupled, as materials mobilized on slopes
are re-stored down-slope or in fans at the base
of slopes (Fryirs and Brierley, 1999; Harvey,
2001; Korup, 2005; Korup et al., 2004).
Tributary systems may be disconnected from
the trunk stream (Ferguson et al., 2006; Rice,
1999). Floodplain pockets and terraces may
disconnect hillslope-derived sediments from
within-channel sediment storage units such
as mid-channel bars and benches (Brierley
and Fryirs, 1999). Limited competence along
a channel may inhibit the movement of
coarse sediment from reach to reach (Hooke,
2003). Alternatively, channel incision may
decouple channel and floodplain processes.
These various forms of decoupling disconnect
large parts of a catchment from the primary
sediment conveyor along the trunk stream,
reducing the catchment area from which
sediments are delivered and transported in
any given period. The position and breaching
capacity of buffers and barriers can be
considered analogous to a series of switches
that determine which parts of the landscape
contribute to the sedimentary cascade over
different time intervals (Brierley and Fryirs,
2009; Fryirs et al., 2007, 2009). To date,
these principles have not been meaningfully
quantified, and they are largely overlooked in
appraisals of catchment-scale sediment flux.
Reach-scale sensitivity to geomorphic
adjustment
As noted from the Lane balance diagram
(Lane, 1955), channels adjust their form in
relation to their sediment loading (volume
and calibre) and flow attributes (the
165
amount of water acting on a given slope).
As such, channel form is determined by the
balance between sediment characteristics
and the ability of the energy regime to
mobilize available materials. Alterations to
these relationships induce aggradation or
degradation along a reach. The nature and
rate of channel adjustments, and associated
sediment flux, vary markedly from bedrock
rivers where erosional processes are dominant,
to alluvial rivers with a mix of erosional and
depositional processes. In essence, bedrock
rivers are sediment-supply-limited systems,
whereas alluvial rivers are typically transportlimited systems (Church, 2006). Alluvial
channels rework sediment stores as they selfadjust along valley floors. As the sensitivity
and capacity for geomorphic adjustment
vary for bedload, mixed load and suspended
load rivers, so too does their ability to store
and rework sediments. The greater the
proportion of readily-accessible stores in the
active channel zone, the higher the likelihood
that these materials will be reworked by
subsequent high flow events.
Sediment organization and roughness
within reaches
Sediment organisation and roughness within
reaches influence sediment-flux at the subreach scale (Hoyle et al., 2008). The nature and
distribution of material within a reach, along
with delivery of sediment from upstream,
determine the amount and calibre of material
available to be transported by a channel.
Sediment flux thus both reflects, and in turn
determines, the nature of sediment stores
and sinks within a reach, and how available
materials are to be transported from one part
of a river system to another. Local availability
of sediment may vary during a given event,
as materials are locally reworked and stored
again at differing stages of flow events. This
is one of many factors that induce hysteresis
effects in sediment loads during floods,
inhibiting our capacity to generate reliable
sediment rating curves.
166
Elements creating resistance, such as
vegetation, wood, bedforms and lagged
gravels, along with channel alignment
itself, further affect bedload mobility, where
channels are presumed to adjust their form to
maximize resistance to prevailing water and
sediment fluxes (e.g., Nanson and Huang,
2008). Sediment mobility varies markedly
for topographic surfaces with differing bed
material attributes and roughness elements
(Mao and Surian, 2010). Sediment sorting,
packing, armour/paving and imbrication
decrease surface sediment mobility, reducing
the frequency of entrainment of bedload
materials (e.g., Haschenburger and Wilcock,
2003). We can differentiate between primary
sources of sediments on hillslopes, short term
stores that are often reworked (e.g., mobile
sediments that make up mid-channel bars)
and long-term sinks that are spatially separated
from reworking processes (e.g., floodplain
and/or terrace deposits; Fryirs and Brierley
2001). The volume, calibre and cohesiveness/
packing of sediments in sources, stores and
sinks, and their positions on the valley floor
(and heights relative to water level), affect the
ease with which these materials are eroded
and/or reworked (Brown, 1987; Davis, 2009;
Heitmuller and Hudson, 2009). Clearly, the
spatial distribution of bed material types
and associated roughness elements exerts
a primary control upon the frequency of
sediment reworking across the valley floor
at different flow stages. Just as importantly,
these reach- and system-specific relationships
change over time. The following section
analyses interactions among these various
controls upon sediment flux in the Tongariro
catchment.
The Tongariro catchment
The influence of landscape setting on
sediment flux
The Tongariro River is classified into nine
River Styles (Fig. 2). Headwater zones of
the catchment are differentiated into two
regions – the Kaimanawa Ranges in the east
and Tongariro National Park to the west. In
the Kaimanawa Ranges, actively uplifting,
poorly consolidated greywacke has created a
dissected landscape with countless steep and
short ‘V’-shaped valleys. First-order streams
are Confined, low sinuosity, gravel-bed rivers.
This landscape unit has a dendritic drainage
pattern. Many small streams drain into the
trunk streams of the Waipakihi River to the
north and Whitikau Stream to the south.
The lower gradient trunk streams flow within
wider valleys with channel sediment stores
such as lateral bars (these are Confined, low
sinuosity, gravel-bed rivers with lateral bars).
In the sparsely vegetated landscapes of the
active volcanic zone of Tongariro National
Park, ephemeral streams driven by rainfall and
snowmelt readily mobilize unconsolidated
Figure 2 – River Styles within the Tongariro Catchment. Grain-size analysis sites are indicated.
167
gravels. Confined, volcanic headwater reaches
flow within gullies. Downstream, shallow
confined valleys cut across the low slope of
the volcanic plateau, with high sediment loads
from lahars, lava flows and tephra deposits
resulting in Confined, low sinuosity, volcanic
plateau cobble-bed rivers (Fig. 2). The flat
terrain disconnects sediment transfer from
the very productive source zone upstream,
acting as a sediment sink. This subcatchment
has a parallel drainage pattern. Streams drain
the volcanic slopes and flow across the central
plateau prior to joining the Tongariro River.
No major tributaries of the size and drainage
area observed within the Kaimanawa Ranges
are found within this subcatchment.
The trunk stream of the Tongariro River is
transitional from a Terrace confined, bedrock
and cobble-bed river to a Partly confined,
wandering cobble-bed river once terraces widen
(Fig. 2). The Tongariro River drains into Lake
Taupo, within a caldera which erupted 1.8 ka
(Manville et al., 2009). The Hatepe eruption
covered the catchment with over 60 km3
of material, of which >30 km3 comprised
ash and pumice tephra deposits (Wilson
et al., 1980). Since then, the channel has
incised into the tephra and underlying lahar
materials (Cronin et al., 1997), resulting in
channels that are partly confined by terraces.
Reworked materials have created a lag deposit
that lines the contemporary channel. Local
gorges have formed in the upper reaches
as the channel has incised into lava flows
(Fig. 3). These rivers have limited accom­
modation space (indicated by discontinuous
floodplain pockets), and a restricted range
of adjustment. Alluvial sediment storage is
restricted to localized wider sections between
the terraces in this sediment transfer zone
(Fig. 2).
Beyond the constraints imposed by terrace
margins, the river becomes an Unconfined,
braided, gravel-bed river (Fig. 2). This reach
is transitional to an Unconfined, meandering
168
sand-bed river and the Unconfined, sand-bed
delta. This area acts as a sediment sink, as the
delta progrades into Lake Taupo.
The primary impact of human disturbance
upon landscape setting, and resulting sediment
flux, is via changes to either sediment inputs
(acceleration or sup­pres­sion) or influences
upon the rate of sediment movement
(primarily induced by hydrological impacts).
Vegetation cover may exert a significant
influence on these controls. Although landuse changes have altered vegetation cover
across much of the Tongariro catchment, there
is no evidence to indicate marked changes
to the transfer of coarse sediments (i.e., the
bedload fraction) for most River Styles in the
catchment. Other than localized adjustments
along the lower course of the river (outlined
below), the pattern of sediment source,
transfer and accumulation zones has not been
altered by human activity. Many landscapes
that are sensitive to human disturbance are
characterized by extensive sediment slugs
triggered by human alterations to sediment
stores on hillslopes and valley floors,
however there are no significant humaninduced ‘legacy’ sediments in the Tongariro
catchment. This highly sediment-charged and
dynamic volcanic landscape has been resilient
to human-induced geomorphic changes.
Along with changes to sediment inputs,
flux relationships are also influenced by
factors that affect the rate of sediment
movement. Two primary human impacts
are evident in the Tongariro catchment. The
impact of dams is discussed in the following
section on landscape connectivity. The
second primary impact is base-level control
brought about by adjustments to the level
of Lake Taupo. Control gates installed in
1941 on the outflow of Lake Taupo regulate
the base level for the Tongariro River. Mean
lake levels from 1906–1940 to 1942–1996
increased by 6.5 cm (Eser and Rosen, 2000),
skewing the overall distribution of lake
Figure 3 – Longitudinal profile of the Tongariro River draining the Waipakihi River in the Kaimanawa
Ranges. Locations for photographs A-F are shown on the long profile. These photographs illustrate
differences in confinement, sediment storage and slope. The Waipakihi River has a relatively low
slope and large bars (A). Downstream the channel narrows and is confined by gorges and bedrock (B).
Dams have been constructed in this area (C). Terrace confinement results in relatively small stores of
active material (D). Moving downstream, bars and islands create a complex river morphology with
sufficient accommodation space for floodplain pockets. Beyond the terraces, the channel becomes
unconfined, slope is reduced and braided channels store a significant volume of sediment.
levels towards higher levels (Smart, 1999).
However, fluctuations in level are not evenly
distributed across seasons. Mean monthly
lake levels display greater differences during
summer months of up to 20 cm higher after
the control gates were installed, while winter
levels are far more similar to those before
artificial control. Lake level variation has also
altered over time, with the mean level from
1942–1949 being 31 cm higher than 1906–
1940. Eser and Rosen (2000) noted that the
extent of the wetland adjacent to the lower
Tongariro River increased notably from 1941
to 1958, increasing the length of the channel
adjacent to wetland. Anecdotal evidence
suggests an increase in flooding, with farming
houses in the delta being abandoned (Smart,
2005). Alterations to the base level are
inferred to have caused an increase in the rate
of deposition and the active channel area as
the channel expanded by over 1 km2 in plan
along a 2.5 km stretch of valley in the braided
reach between 1928 and 1958 (discussed
later). The operation of the Taupo Control
169
Gates also results in a greater capacity to
convey water than the original natural outflow,
however the level is managed in accord with
seasonal patterns of power usage. Despite the
increased capacity of the control gates, there
are still periods when inflows exceed outflows,
leading to higher lake levels. This contributed
to the extent of flooding and deposition in
the 2004 flood (Munro, 2005). The increase
in flooding may also be influenced by natural
subsidence in the delta (Genesis Power
Limited (2000) estimate subsidence rates of
2 mm/year following earthquakes in 1983).
These are essentially localized adjustments,
and the volcanic history continues to exert
the dominant imprint upon sediment flux.
Landscape connectivity
There is abundant sediment in the Tongariro
catchment. Volcanic events generate irregular
and dramatic influxes of sediment, and
incised streams in upland areas rework these
materials. Extensive accommodation space
atop the Central Plateau acts as a major
sediment sink in the middle-upper part of the
basin. Downstream of various mid-catchment
gorges, the lower course of the Tongariro
River has incised into volcanic materials,
creating a partly-confined valley that is inset
between terraces (cf., Fryirs and Brierley,
2010; note that the terraces themselves are
long-term sediment sinks). There is limited
accommodation space for sediment storage
in this part of the catchment, and the reach
acts as a transfer zone. Beyond the terraces,
however, contemporary sediment storage is
high, marked by in-channel sediment stores
(braid bars) and floodplain sinks.
Various landforms impede sediment
conveyance through the Tongariro catchment
(Fig. 4). Essentially, the upper and lower
catchment are disconnected by base-level
controls induced by the bedrock gorge in
mid-catchment (Fig. 3). This acts as a barrier
to sediment conveyance, with significant
storage of materials in upstream sediment
170
sinks. Although hillslope-channel coupling is
highly effective in the steep, dissected terrain
of the upper catchment, and large volumes
of sediment are delivered to the valley floor,
most of this material is retained within the
flat terrain of the plateau landscape.
The marked discontinuity in sediment
flux along the Tongariro River is reflected
by downstream changes in bed material
size. Above the terrace-constrained reach
of the mid-lower catchment, the median
bed material size (D50) (Wolman count) in
the Waipakihi Stream is 61 mm. There is
significant in-channel sediment storage, with
large areas of active gravel stored as lateral bars
(the channel is 300 m at the widest point)
(Fig. 3, photograph A). In contrast, the upper
section of the terrace-constrained reach,
where lahar materials are reworked within the
incised channel trench, has a D50 of 230 mm.
Bars are not as large and continuous as in the
Waipakihi Stream (Fig. 3, photograph E).
Median bed material size in the braided reach
is 85 mm. The volumes of gravel delivered
and transported through the terrace sections
are ultimately captured in the braided section,
making this reach particularly sensitive to
changes in sediment flux. There is limited
conveyance of gravels beyond the braided
reach, and sand-sized materials dominate
the meandering sand-bed river and delta
downstream (Fig. 3, photograph F). Sediment
trapping within these lower reaches partially
disconnects sediment transfer to Lake Taupo,
which itself is the ultimate sediment trap
on the Tongariro system. Average rates of
sediment accumulation in the delta zone
for the past 1850 years are estimated to be
around 2.6 million tonnes a year (Smart,
2005). Contemporary rates are around 5% of
this. This reflects a decrease in both sediment
supply and catchment-wide connectivity.
Two dams have been constructed within
the imposed, confined section of the Terrace
confined, bedrock and cobble-bed river (Fig. 3,
photograph C). Flow downstream of these
structures is regulated, with the mean flow
reduced by 40%. Sediment delivery through
sluice gates occurs only during large events.
Sluice gates on the Rangipo Dam are raised
and the Poutu Intake is shut off (so all flow is
delivered to the river) during high magnitude
events (>100 m3s-1), scouring up to 60,000
tons of sediment from the Rangipo Reservoir
(Collier, 2002). The gates remain open until
all the sediment has been scoured from the
settling basin, which takes around 8 hours.
Natural flows are retained in the river for a
few days following dam flushing to allow
continued transport of the sediment mobilized
from the dam (Genesis Power Limited,
2000). The Poutu Intake (downstream of the
Rangipo) allows suspended and bed sediment
to be transported through a 200 mm screen
into settling chambers which are scoured
approximately once a week (Jowett, 1980).
As such, sediment delivery
and transport through the
mid-catchment is irregular,
with surges in supply
following flood events > 100
m3s-1 and reduced loads at
normal flows. Despite these
alterations to the delivery
of material to the lower
catchment, contemporary
operation of the Tongariro
Power Scheme appears to
have had a minimal impact
upon the contemporary
bedload sediment flux
and associated channel
adjustment. The large bed
material derived from lahars
requires high magnitude
events to mobilize and
rework much of the channel
within the terraces, creating
a resilient system.
Stopbanks constructed in
the lower reaches adjacent to
Figure 4 – Sediment connectivity throughout the Tongariro Turangi artificially confine
Catchment. Labels A to F correspond with the photos and the river, which limits the
locations shown on the long profile in Figure 3. Connectivity capacity for geomorphic
was assessed with regard to landscape unit characteristics and
adjustment. Davies and
the distribution of River Styles across the catchment and their
character and behaviour (Fryirs et al., 2007). This involved two McSaveney (2006) have
components: hillslope-channel connectivity (i.e., higher dissected linked stopbanks and flood­
confined valleys can deliver high sediment loads to the channel works to channel aggrada­
easily) and longitudinal channel connectivity – the ability of tion. However, stopbanks
the channel to deliver the sediment supplied to it (i.e., the along the lower Tongariro
braided reach stores the gravel sized fraction delivered, making River are located along
it a buffer).
171
Figure 5 – Planform adjustment for different River Styles within the lower Tongariro
River over the past 80 years extracted from a survey map (1928) and aerial photography
(1958-2007).
relatively narrow and straight reaches
that have undergone little change in
total channel area in the past 80 years
(Fig. 4). This suggests these reaches are well
connected, since sediment delivered to them
is transported rather than stored in bars. The
stopbanks may therefore maintain or improve
a high degree of longitudinal connectivity in
this section of the system, flushing sediment to
the unconfined braided reach downstream.
172
Reach-scale sensitivity to geomorphic
adjustment along the lower Tongariro River
Planform adjustments along the lower
Tongariro River over the last 80 years are
shown in Figure 5. The Partly-confined,
wandering, cobble-bed reach has incised into
tephra and lahar materials and has limited
capacity for adjustment. A coarse basal lag
inhibits further incision (Fig. 6). Terraces
constrain channel migration, fixing bends
in place and forcing river morphology and
bar turn­over. Wider valley sec­tions with
floodplain pockets are more sensitive to
adjustment, as they are prone to reworking
by activation of flood channels and flood­
plain stripping. This creates lateral and
mid-channel bars. This reach efficiently
conveys materials delivered from upstream.
The extent of incision and the height of
terraces progressively but gradually decreases
downstream. This reach has experienced
negligible adjustments over the past 80 years,
with no evidence for sustained aggradation or
degradation over this period (Smart, 1999).
No notable planform changes have occurred
in response to flow adjustments associated
with operation of the dams.
The Unconfined, braided, gravel-bed river
that lies immediately beyond the terraces
has formed through long-term deposition
of sediments generated by the reworking of
terrace materials. The unconfined nature of
the reach and low slope results in uncon­
strained channel adjustment and aggradation.
This bedload-dominated river is prone to
rapid realignments of channels via thalweg
shift or downstream and lateral migration
of bars. Non-cohesive channel boundaries
are readily reworked. Extensive sediment
stores are reworked over annual or decadal
timescales, as evidenced by recurrent planform
adjustments (Fig. 5).
Despite their non-cohesive sandy
boundaries, the Unconfined, meandering,
sand-bed river and the multi-channelled delta
at the lower reach of the Tongariro have shown
limited planform adjustment over the past
80 years (Fig. 4). The low slope, unconfined
valley setting and low channel capacity
result in extensive sediment storage on the
floodplain, delta and associated wetlands of
this sediment sink. Palaeo-channels on the
floodplain indicate a history of avulsion.
Limited channel adjustments indicate
a lack of lateral reworking of materials.
Ongoing channel contraction has decreased
channel capacity and infilled pools, with an
average decrease in channel width of 26 m.
Colonization by exotic willow species over
the past 40 years has increased bank strength.
Removal of these trees at isolated locations
over recent years has led to bank erosion,
although as yet little significant change in
width and channel planform.
Variability in reach-scale geomorphic
adjustment, and resulting reworking of valley
floor materials and sediment flux, reflect
the sensitivity of the reach to adjustments
on the one hand, and the input of materials
into that reach as determined by landscape
connectivity on the other (see Fig. 1).
Given its unconfined nature, the braided
reach beyond the terraces is more prone to
adjustment, while the meandering and delta
reaches act as long-term sediment sinks.
In terms of human impacts on sensitivity to
adjustment, gravel extraction in the braided
reach in the 1960s and 1970s resulted in a
single channel with low sinuosity, thereby
controlling channel expansion. Subsequently,
large floods transported more material into
this reach, instigating a braided planform
(Fig. 5). River responses to other management
activities, including stopbank construction
and control of Lake Taupo levels, are not easy
to discern along the lower Tongariro River.
Sediment organization and roughness along
the lower Tongariro River
Variability in channel planform, geometry,
bed material and vegetation associations are
shown for three reaches of the lower Tongariro
River in Figure 6. Adjustments within the
Partly confined, wandering cobble-bed reach
are characterized by a cycle of mid-channel
and lateral bar formation from reworking of
floodplain surfaces, stabilization by vegetation
and transition back to floodplain. The sedi­
ment distribution is heterogeneous and
bimodal within this reach, reflecting the coarse
lahar lag and the smaller-sized active fraction
delivered from upstream. Large protruding
173
Figure 6 – Planform maps, cross-sections, reach-scale photographs (showing vegetation) and grainsize distribution (100 clast Wolman counts for the coarsest surface) for three River Styles within
the Tongariro River. Distance bars on the cross-sections refer to horizontal distances only, as the
vertical scale has been exaggerated. The Unconfined, meandering, sand-bed river has localised gravel
accumulations on point bars.
174
clasts protect smaller materials, increasing the
shear stress required to mobilize them and
decreasing the frequency of reworking. These
inherited materials are locally reworked, and
patterns of bed material organization likely
reflect downstream changes in the width
of the active channel zone within terraces
(cf., Cowie and Brierley, 2008).
Bed material size is notably finer and more
homogenous (better sorted) in the braided
reach that lies downstream of the terraces
(Fig. 6). Mobile channels rework loosely
consolidated, unpacked material on braid
bars with little vegetation. Limited grain
protrusion and hiding results in limited
roughness. Adjacent floodplain sections act
as sediment sinks. The uniform sand-sized
material in the meandering and delta reaches
limits grain resistance. However, sinuosity and
channel multiplicity induce form resistance
that promotes deposition. In addition,
planted exotic willows provide instream
roughness, locally trapping sediments.
Human disturbance has had a limited
and localized impact upon bed material
organization along the lower Tongariro River.
Long-term incision into volcanic materials
has been the primary determinant of available
materials and their distribution, fashioning
the pronounced downstream gradation from
cobble lag materials through well-sorted gravels
to sand deposits. Dams have interrupted
sediment conveyance, but there is a sufficient
range of flows to ensure that disruption to
sediment patterns is not significant. Local
impacts are most pronounced in the braided
reach, where stopbanks and gravel extraction
have disturbed the channel bed and altered
the ease of sediment conveyance. Also,
clearance of riparian vegetation and an influx
of willows have affected channel geometry
along the meandering sand-bed and multichanneled delta reaches. In longer-term
perspective, these factors have exerted a
negligible impact upon sediment flux in the
catchment as a whole.
Summary and implications for river
management
The nature and rate of sediment flux in the
Tongariro Catchment are largely a product of
the volcanic terrain. The legacy here is one
of interrupted sediment availability, with
volcanic eruptions and major reworking
events such as lahars resetting the sediment
balance in an irregular manner (i.e., landscape
setting). River channels have adapted to this
irregular sediment input. Incision through the
tephra and lahar deposits has created a partly
confined valley in which the channel is inset
within terraces. Critically, the contemporary
input of sediment into the lower course of
the river is limited by the disconnected nature
of the landscape upstream (i.e., landscape
connectivity). The volcanic plateau acts as
a major buffer to sediment conveyance.
In addition, base-level control induced by
gorges acts as a major barrier to downstream
movement of sediment (Fig. 3).
Downstream changes in valley confinement
and slope cause significant variability in river
adjustment along the lower course of the
Tongariro River. The channel is laterally
constrained within terrace materials for
much of the lower course. There is a marked
transition in river type and associated
patterns of sediment flux in the braided reach
beyond the terraces. In contrast to all other
reaches along the lower Tongariro River,
this reach has been subjected to significant
channel planform adjustments over the last
80 years (i.e., this reach is more sensitive to
geomorphic adjustment). Interestingly, this
reach has behaved as a sediment trap, limiting
conveyance of gravel materials to the sandbed meandering and multi-channel delta
reaches downstream. The latter reaches have
experienced negligible planform adjustments
over the last 80 years, but palaeo-channels
indicate an avulsive history for this area.
The inter-related larger-scale geomorphic
controls affect bed material organization and
the ease of sediment reworking along the
175
channel. As the channel incised to create the
terraces within the partly confined reach,
it selectively retained the coarsest fraction
of lahar deposits as basal lag materials
(cobbles) that are infrequently reworked.
However, selectively entrained finer gravels
have been flushed downstream, presenting
a completely different set of uniformly
sized, readily mobilized materials within the
braided reach (Fig. 6). Braid bars are readily
reworked, bringing about recurrent planform
adjustments. Sediment trapping by this reach
has seemingly inhibited the extent and rate
of channel adjustments in the downstream
sand-bed reaches.
The imprint of the volcanic setting upon
sediment flux in the Tongariro Catchment
is so prominent that system responses to
human disturbance have been limited. High
sediment availability, along with a naturally
sparse vegetation cover across much of
the catchment, have minimized responses
to land-use change. Although dams have
altered the flow regime of the river, impacts
upon sediment flux have been well managed
and no major changes to channel planform
are evident. The increase in the frequency
of high water levels in Lake Taupo causes a
back-water effect, reported to extend 3 km
upstream of the delta, enhancing landward
movement of saturated soils and the area
of wetland (Tonkin and Taylor, 1999; Eser
and Rosen, 2000). Increased flooding and
aggradation may have affected the extent of
deposition and braiding, especially following
the installation of the control gates in 1941
(Smart, 1999; Fig. 4). As a result, the primary
sediment management issue in this catchment
relates to ongoing adjustments in the braided
reach, and associated management strategies
via stopbanks and gravel extraction. In a sense,
stopbanks can transfer problems downstream
and require ongoing maintenance. Unless
gravel extraction is undertaken in a sustainable
manner, problems will ensue and channel
adjustments are likely to be accelerated (with
176
associated implications for aquatic habitat).
Alternative approaches to river management
apply ‘space to move’ or ‘erodible corridor’
concepts, striving to allow the river to selfadjust, thereby minimizing the need for direct
management interventions and associated
costs (see Rapp and Abbe, 2003; Piegay
et al., 2005).
Conclusion
This study highlights the role of geomorphic
controls on the spatial and temporal variability
of sediment availability across a landscape,
and the rate at which those materials move
through river systems. Variability in sediment
supply draws into question the applicability
of modelling procedures that assess sediment
flux under an assumed set of near constant
(uniform) conditions. Unless geomorphic
considerations are incorporated within
engineering practices, management outcomes
are likely to be compromised. Effective
management of sediment flux cannot be
undertaken at cross-section to reach scales.
Instead, a landscape-scale view of sediment
continuity is required. This must incorporate
an understanding of sediment generation
and movement on hillslopes, effectiveness of
sediment transfer to valley floors, the efficiency
of downstream conveyance of sediments
and their storage along valley floors. These
interactions are complex – some river systems
are highly sensitive to change, and disturbance
responses are readily conveyed through a
catchment. Other systems are remarkably
resilient; change seldom occurs, and any
adjustments that do take place have negligible
implications for other parts of that catchment
(i.e., some landscapes are naturally connected;
others not). The Tongariro catchment is
quite resilient. Human disturbance has not
had a profound impact upon sediment flux
in this river system. The primary sediment
management issue in this catchment relates
to ongoing adjustments in the braided reach,
and associated management via stopbank
construction and gravel extraction.
Acknowledgements
Fruitful discussions with the GEOG 739
(bedload transport) class taught by GB and
SC as part of the Fluvial Processes Initiative
at the University of Auckland helped in the
development of this manuscript. Constructive
review comments by Ian Fuller and an
anonymous reviewer are greatly appreciated.
The support of the University of Auckland
Cross-faculty Research Initiatives Fund
(Grant 3624991) is gratefully acknowledged.
References
Adams, J. 1979: Sediment loads of North Island
Rivers, New Zealand - a reconnaissance.
Journal of Hydrology (NZ) 18: 36-48.
Ballantyne, C.K. 2002: Paraglacial geomorphology.
Quaternary Science Reviews 21: 1935-2017.
Brasington, J.; Richards, K. 2007: Reducedcomplexity, physically-based geomorphological
modelling for catchment and river management.
Geomorphology 90: 171-177.
Brierley, G.J. 2010a: Landscape memory: The
imprint of the past on contemporary landscape
forms and processes. Area 42: 76-85.
Brierley, G.J. 2010b: The imprint of landscape
memory upon catchment-scale sediment
budgets. International Journal of Erosion Control
Engineering 3: 4-8.
Brierley, G.J; Fryirs, K.A. 1999: Tributary-trunk
stream relations in a cut-and-fill landscape:
A case study from Wolumla catchment, New
South Wales, Australia. Geomorphology 28: 6173.
Brierley, G.J; Fryirs, K.A. 2005: Geomorphology
and River Management: Applications of the
River Styles Framework. Blackwell Publishing,
Sydney, Australia.
Brierley, G.J; Fryirs, K.A. 2009: Don’t fight the
site: Three geomorphic considerations in
catchment-scale river rehabilitation planning.
Environmental Management 43: 1201-1218.
Brierley, G.; Fryirs, K.; Jain, V. 2006: Landscape
connectivity: the geomorphic basis of
geomorphic applications. Area 38: 165-174.
Brooks, A.P.; Brierley, G.J. 2004: Framing realistic
river rehabilitation programs in light of altered
sediment transfer relationships: lessons from
East Gippsland, Australia. Geomorphology 58:
107-123.
Brown, A.G. 1987: Long-term sediment storage
in the Severn and Wye Catchments. In:
Gregory, K.J.; Lewin, J.; Thornes, J.B. (eds.)
Palaeohydrology in Practice. Wiley, Chichester:
307-332.
Church, M. 2006: Bed material transport and the
morphology of alluvial river channels. Annual
Review of Earth and Planetary Science 34: 325354.
Church, M.; Slaymaker, O. 1989: Disequilibrium
of Holocene sediment yield in glaciated British
Columbia. Nature 337: 452-453.
Collier, K.J. 2002: Effects of flow regulation
and sediment flushing on instream habitat
and benthic invertebrates in a New Zealand
River influenced by a volcanic eruption. River
Research and Applications 18: 213-226.
Cowie, M.; Brierley, G.J. 2008: The influence
of lateral confinement upon the downstream
gradation in grain size of the Lower Ngaruroro
River, New Zealand. The Open Geology Journal
2: 46-63.
Cronin, S.J.; Neall, V.E.; Palmer, A.S. 1997:
Lahar history and hazard of the Tongariro
River, northeastern Tongariro Volcanic Centre,
New Zealand. New Zealand Journal of Geology
and Geophysics 40: 383-393.
Davies, T.R.; McSaveney, M.J. 2006: Geomorphic
constraints on the management of bedloaddominated rivers. Journal of Hydrology (New
Zealand) 45(2): 111-130.
Davis, L. 2009: Sediment entrainment potential
in modified alluvial streams: Implications for
Re-mobilization of stored in-channel sediment.
Physical Geography 30: 249-268.
Eser, P.; Rosen, M.R. 2000: Effects of artificially
controlling levels of Lake Taupo, North Island,
New Zealand, on the Stump Bay wetland.
New Zealand Journal of Marine and Freshwater
Research 34: 217-230.
Ferguson, R.I.; Cudden, J.R.; Hoey, T.B.; Rice,
S.P. 2006: River system discontinuities due
to lateral inputs: Generic styles and controls.
Earth Surface Processes and Landforms 31:
1149-1166.
177
Fryirs, K.A.; Brierley, G.J. 1999: Slope-channel
decoupling in Wolumla catchment, New
South Wales, Australia: The changing nature
of sediment sources following European
settlement. Catena 35: 41-63.
Fryirs, K.A.; Brierley, G.J. 2001: Variability in
sediment delivery and storage along river
courses in Bega catchment, NSW, Australia:
implications for geomorphic river recovery.
Geomorphology 38: 237-265.
Fryirs, K.A.; Brierley, G.J. 2010: Antecedent
controls on river character and behaviour in
partly confined valley settings: Upper Hunter
catchment, NSW, Australia. Geomorphology
117: 106-120.
Fryirs, K.A.; Brierley, G.J.; Preston, N.J.; Kasai,
M. 2007: Buffers, barriers and blankets: The
(dis)connectivity of catchment-scale sediment
cascades. Catena 70: 49-67.
Fryirs, K., Spink, A.; Brierley, G. 2009: PostEuropean settlement response gradients of
river sensitivity and recovery across the upper
Hunter catchment, Australia. Earth Surface
Processes and Landforms 34: 897-918.
Genesis Power Limited 2000: Tongariro Power
Development: Resource consent applications and
assessment of Environmental Effects, Genesis
Power Limited, Turangi.
Griffiths, G.A. 1979: High sediment yields from
major rivers of the Western Southern Alps,
New Zealand. Nature 282: 61-63.
Griffiths, G. 1982: Spatial and temporal variability
in suspended sediment yields of North Island
Basins, New Zealand. Water Resources Bulletin
18: 575-584.
Harvey, A.M. 2001: Coupling between hillslopes
and channels in upland fluvial systems:
implications for landscape sensitivity illustrated
from the Howgill Fells, northwest England.
Catena 42: 225-250.
Harvey, A.M. 2002: Effective timescales of
coupling within fluvial systems. Geomorphology
44: 175-201.
Haschenburger, J.K.; Wilcock, P.R. 2003: Partial
transport in a natural gravel bed channel. Water
Resources Research 39: 1-9.
Heitmuller, F.T.; Hudson, P.F. 2009: Downstream
trends in sediment size and composition of
channel-bed, bar, and bank deposits related
to hydrologic and lithologic controls in the
Llano River watershed, central Texas, USA.
Geomorphology 112: 246-260.
178
Hicks, D.M.; Gomez, B.; Trustrum, N.A. 2000:
Erosion thresholds and suspended sediment
yields, Waipaoa River Basin, New Zealand.
Water Resources Research 36: 1129-1142.
Hicks, D.M.; Hill, J.; Shankar, U. 1996: Variation
of suspended sediment yields around New
Zealand: the relative importance of rainfall and
geology. International Association of Hydrological
Sciences Publication 236: 149-156.
Hooke, J. 2003: Coarse sediment connectivity in
river channel systems: a conceptual framework
and methodology. Geomorphology 56: 79-94.
Hoyle, J.; Brierley, G.J.; Brooks, A.; Fryirs, K.
2008: Gravel organisation along the Upper
Hunter River, NSW. In: Habersack, H., Piégay,
H. and Rinaldi, M (eds.) Gravel Bed Rivers 6.
From Process Understanding to River Restoration.
Elsevier, Amsterdam: 419-441.
Hudson, P.F. 2003: Event sequence and sediment
exhaustion in the lower Panuco Basin, Mexico.
Catena 52: 57-76.
Jain, V.; Tandon, S.K. 2010: Conceptual assessment
of (dis)connectivity and its applications to the
Ganga River dispersal system. Geomorphology
118: 349-358.
James, L.A. 2010: Secular sediment waves, channel
bed waves, and legacy sediments. Geography
Compass 4: 576-598.
Jowett, I.G. 1980: Sediment and the operation of
the Poutu Intake: Tongariro Power Development.
Power Division, Ministry of Works and
Development, Wellington.
Korup, O. 2005: Large landslides and their effect
on sediment flux in South Westland, New
Zealand. Earth Surface Processes and Landforms
30: 305-323.
Korup, O.; McSaveney, M.; Davies, T.R.H. 2004:
Sediment generation and delivery from large
historic landslides in the Southern Alps, New
Zealand. Geomorphology 61: 189-207.
Lane, E.W. 1955: The importance of fluvial
morphology in hydraulic engineering.
Proceedings of Hydrological Division. ASCE
81, paper 745: 1-17.
Lane, S.N.; Reid, S.C.; Tayefi, V.; Yu, D.; Hardy,
R.J. 2008: Reconceptualising coarse sediment
delivery problems in rivers as catchment-scale
and diffuse. Geomorphology 98: 227-249.
Lane, S.N.; Richards K.S. 1997: Linking river
channel form and process: time, space and
causality revisited. Earth Surface Processes and
Landforms 22: 249-260.
Manville, V.; Segschneider, B.; Newton, E.; White,
J.D.L.; Houghton, B.F.; Wilson, C.J.N. 2009:
Environmental impact of the 1.8ka Taupo
Eruption, New Zealand: Landscape responses
to a large-scale explosive rhyolite eruption.
Sedimentary Geology 220: 318-336.
Mao, L.; Surian, N. 2010: Observations on
sediment mobility in a large gravel bed river.
Geomorphology 114: 326-337.
Merritt, W.S.; Letcher, R.A.; Jakeman, A.J. 2003:
A review of erosion and sediment transport
models. Environmental Modelling and Software
18: 761-799.
Montgomery, D.R.; Panfil, M.S.; Hayes, S.K.
1999: Channel-bed mobility response to
extreme sediment loading at Mount Pinatubo.
Geology 27: 271-274.
Munro, A. 2005: Taupo, Waikato and Waipa
Management zones leap day flood event; February
29 to March 5, 2004. Environment Waikato
Technical Report 2004/06, Environment
Waikato, Hamilton.
Nanson, G.C.; Huang, H.Q. 2008: Least
action principle, equilibrium states, iterative
adjustment and the stability of alluvial
channels. Earth Surface Processes and Landforms
33: 923–942.
Page, M.; Marden, M.; Kasai, M.; Gomez, B.;
Peacock, D.; Betts, H.; Parkner, T.; Marutani,
T.; Trustrum, N. 2007: Changes in basinscale sediment supply and transfer in a rapidly
transformed New Zealand landscape. In:
Habersack, H.; Piégay, H.; Rinaldi, M (eds.)
Gravel Bed Rivers 6. From Process Understanding
to River Restoration. Elsevier, Amsterdam:
337-356.
Phillips, J.D. 2003: Alluvial storage and the longterm stability of sediment yields. Basin Research
15: 153-163.
Phillips, J.D. 2007: The perfect landscape.
Geomorphology 84: 159-169.
Piégay, H.; Darby, SE.; Mosselman, E.; Surian, N.
2005: A review of techniques available for
delimiting the erodible river corridor: a sus­
tainable approach to managing bank erosion.
River Research and Applications 21: 773-789.
Rapp C.; Abbe T. 2003: A framework for delineating
channel migration zones. Ecology Publication
#03-06-027. Washington State Department of
Ecology. Washington.
Reid, L.M.; Dunne, T. 1996: Rapid Evaluation of
Sediment Budgets. Catena Verlag, Reiskirchen.
Rice, S. 1999: The nature and controls on
downstream fining within sedimentary links.
Journal of Sedimentary Research 69: 32-39.
Richards, K. 2002: Drainage basin structure,
sediment delivery and the response to
environmental change. In: Jones, S.J.; Frostick,
L.E. (eds.) Sediment Flux to Basins: Causes,
Controls and Consequences. The Geological
Society of London Special Publication 5(191):
149-160.
Schumm, S.A. 1977: The Fluvial System. John
Wiley and Sons, New York.
Sear, D.A. 1994: River restoration and
geomorphology. Aquatic Conservation: Marine
and Freshwater Ecosystems 4: 169-177.
Sear, D.A.; Newson, M.D.; Brookes, A. 1995:
Sediment-related river maintenance: The
role of fluvial geomorphology. Earth Surface
Processes and Landforms 20: 629-647.
Simon, A. 1992: Energy, time, and channel
evolution in catastrophically disturbed fluvial
systems. Geomorphology 5: 945-372.
Smart, G.M. 1999: Lower Tongariro flooding
and erosion study. N.I.W.A. Client Report
CHC99/49 for Genesis Energy. Christchurch.
Smart, G. 2005: The Higher Lower Tongariro.
Environment Waikato Technical Report
2005/49, Environment Waikato, Hamilton.
Tonkin & Taylor 1999: Lower Tongariro River
Natural Hazard Management Plan. Tonkin and
Taylor Report Reference No.12310.
Vörösmarty, C.J.; Meybeck, M.; Fekete, B.;
Sharma, K.; Green, P.; Syvitski, J.P.M.
2003: Anthropogenic sediment retention:
major global impact from registered river
impoundments. Global and Planetary Change
39: 169-190.
Wilkinson, B.H.; McElroy, B.J. 2007: The
impact of humans on continental erosion and
sedimentation. Geological Society of America
Bulletin 119: 140-156.
Wilson, C.J.N.; Ambraseys, N.N.; Bradley, J.;
Walker, G.P.L. 1980: A new date for the
Taupo Eruption, New Zealand. Nature 288:
252-253.
179
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