Te Hoiere River Dynamics
River processes and typology in the Te Hoiere and Rai catchments
The Drainage Network
The Te Hoiere catchment lies between Blenheim and Nelson, within the Marlborough Sounds. The catchment extends over approximately 890 km 2 , with the inclusion of the Rai (to the northeast) and Whakamarino (to the southeast; previously Wakamarina) catchments. The Pelorus/Te Hoiere discharges into the Havelock estuary and Pelorus Sound. The Te Hoiere mainstem and the Whakamarino drain the northern side of the Richmond Ranges, while the Rai rises in the Bryant range to the northeast. The adjacent Kaituna River also discharges into the Havelock estuary.
The basin has undergone large-scale tilting and deformation over geologic time, leading to a relative fall in base-level for the river, exposing bedrock in the major valleys. This geological activity has lead to major shifts in the drainage pattern over time. These include sea inundation of the Pelorus sound, reversal of drainage direction of the Kaituna River and changes in the morphologic character of the rivers themselves.
The longitundinal profile of the Te Hoiere River reveals the gradual transition from very steep (>5%) channel gradients in the headwater to the much more subdued lower river (0.04%)
The Rai River (blue catchment) has three prominent upper tributaries: the Rongo, Tunakino and Opouri streams. These rivers have relatively subdued slopes (less than 2%, as low as 0.2%). This is due in part to the tectonic back-tilting of the basin, leading to different river character than the rivers to the south and west.
In contrast, the Wakamarino River (green catchment) has very steep headwaters, exceeding 5-7% in places.
This increased steepness means the river has energy to transport sediment out of the catchment and erode down to bedrock.
The Kaituna River (orange catchment) also shows quite steep headwater tributaries joining the more subdued gradient in the mainstem river. The Kaituna river valley was formed by the much larger paleo-Pelorus river when it drained what is now the Pelorus and Queen Charlotte Sounds. The entire Marlborough sounds area was drained by this pre-historic river into the Wairau River. This explains the low slope and over-wide nature of the valley floor, relative to the size of the current river.
There are a large number of steep tributary systems that deliver coarse-grained sediment to the major river systems.
The river profiles shows a number of knickpoints: discontinuities in the bed slope that, in this case, show the influence of the bedrock substrate on the local evolution of the channel. This gives rise to a diversity of river gradient and form, such as waterfalls and cascades, as well as deep pools (photo below).
Cascading sections reveal outcrops of more resistant bedrock at several points along the river; these sites emerge as 'knickpoints' in the river long profile.
The Marlborough landscape evolved by drainage rearrangement from mountain uplift, strike-slip faulting and the headward erosion of mainstem rivers during the Kaikoura Orogeny, some 25 million years ago. The bedrock is predominantly Mesozoic siliceous greywackes and schists, with bands of serpentinite greywacke (Walls & Laffan, 1986). Weathering of weaker mineral bands in the schist produces planes of weakness prone to deep and surficial slippage with sediment detritus of characteristically flat (platy) form. Overlying the schist is a layer of hardened sandstones and siltstones as greywacke and argillite atop the Te Hoiere Group (Lauder, 1987).
Te Hoiere NZ Geology. See GNS Science for legend, source material and other details.
The river's drainage has been dramatically influenced by differential, subsiding tectonic movement. Prior to the Last Interglacial, it is thought the Te Hoiere River flowed south, into the Wairau River instead of east and north into Pelorus Sound as it does now. The change in drainage direction reflects northward regional tilting (Mortimer and Wopereis, 1997; Craw et al., 2007). The build-up of a thick (~60 m) sedimentary sequence in the Kaituna and Are Are valleys (to the east) prompted the overtopping of a subsiding drainage divide farther north, in what is now Pelorus Sound. Subsequent erosion through this divide was probably facilitated by shoreline retreat and associated downcutting during sea level high-stands of the Last Interglacial.
Landslides, Blankets, Fans and Terraces
In the rugged topography of the Bryant and Richmond ranges, landslides, rockfall and debris flows are part of the drainage network evolution. These provide important source materials for rivers, as well as key river boundary conditions along the valley walls.
Many ancient bedrock failures can be found along the valley walls. These can be difficult to see in aerial images, but they are revealed with high-resolution LiDAR topographic surveys. These are highlighted in the map below, along with deposits of colluvium that have accumulated at the base of the valley walls.
In the map, headscarp zones are indicated in orange and rockfall debris is shown in purple. Blankets of colluvium at the base of slope are highlighted in green. Fans are found at the outlet of constricted valleys or gullies, where debris is deposited in lobate, semi-circular deposits. Both river and colluvial fans may coalesce, making it difficult to distinguish these deposits from blankets.
River Variation and Variability
River morphology is largely shaped by valley gradient, sediment supply, and the size of the sediment (boulders, gravel, sands, silt). Rivers also respond to the relative confinement by valley walls and other boundary influences (above). As rivers flow from mountains to the coast, the factors that influence their form vary systematically. There are various ways of classifying river typology - one of these is River Styles (e.g. Brierley and Fryirs, 2018). A River Styles map of the catchment is shown below, providing a picture of the basin-wide variation in river morphology in one map. The map legend is shown below.
Te Hoiere River Styles Map
River Styles: legend of different river types found in the Te Hoiere catchment.
Floodplains and Valley Fills
The sedimentary fills within the valleys hold a record of past events, and the surface features record past evolution of the river channel (see below). By mapping out the landforms (e.g., fans, floodplains, terraces), the modern trajectory of the river becomes clearer, and it is possible to better assess the likely future path of the river. Floodwater tends to spill across the lower-elevation floodplains, or it may be constrained by high terraces. Fans represent the dynamic interface between coarse-grained sediments emerging from tributaries with those travelling along the river channel. Detailed geomorphic maps such as these can be helpful for planning more resilient infrastructure and property along the length of the river.
Looking downstream on the Opouri River
In the mid-reaches of the Lower Te Hoiere mainstem river. A number of abandonned meander loops are evident. These are occasionally re-activated in flood conditions.
In the mid-reaches of the Lower Te Hoiere mainstem river. A number of abandonned meander loops are evident. These are occasionally re-activated in flood conditions.
The Evolution of the River
The meandering rivers of the Te Hoiere catchment are evolving continually, with every flood event leading to incremental erosion along the channel. By looking back over decades, it is possible to reconcile the modern erosion we see with the very long-term trajectory of meandering across the floodplain (see mapping, above). The colours in the Relative Elevation Model (REM), below are keyed to the topographic elevation above the river bed, emphasising the minute variations (abandoned channels, scroll features, oxbows) associated with past erosion events - much of the post-glacial history of the channel is recorded in the floodplain topography. Overlaid on this map is the 1960 river boundary (blue polygon) and model time-steps of meander evolution according to a numerical model (Bogoni et al., 2017; Ikeda et al., .198): this provides an approximate picture of the likely trajectory of river meanders: there are many possible contingencies that alter these predictions, and the timescale is quite uncertain. The intensity and frequency of flooding will greatly influence the pace of meander evolution. The tighter meander bends are the ones most likely to exhibit dynamic changes.
Historic imagery (1959-1960) at left, and the Relative Elevation Map (REM) at right. A model of river evolution is overlaid on the REM, outlining the potential direction of future meander movement. The timescale is uncertain. However, swiping back and forth, one can obtain a sense of the overall trajectory of the river over decades to centuries.
References
Bogoni, M., Putti, M., & Lanzoni, S. (2017). Modeling meander morphodynamics over self‐formed heterogeneous floodplains. Water Resources Research, 53(6), 5137-5157.
Borselli, L., Cassi, P., & Torri, D. (2008). Prolegomena to sediment and flow connectivity in the landscape: A GIS and field numerical assessment. Catena, 75(3), 268-277.
Craw, D., Anderson, L., Rieser, U., & Waters, J. (2007). Drainage reorientation in Marlborough Sounds, New Zealand, during the last interglacial. New Zealand Journal of Geology and Geophysics, 50(1), 13-20.
Fryirs, K. A., & Brierley, G. J. (2018). What’s in a name? A naming convention for geomorphic river types using the River Styles Framework. PloS one, 13(9), e0201909.
Ikeda, S., Parker, G., & Sawai, K. (1981). Bend theory of river meanders. Part 1. Linear development. Journal of Fluid Mechanics, 112, 363-377.
Mortimer, N., & Wopereis, P. (1997). Change in direction of the Pelorus River, Marlborough, New Zealand: evidence from composition of Quaternary gravels. New Zealand Journal of Geology and Geophysics, 40(3), 307-313.
Walls, G. Y., & Laffan, M. D. (1986). Native vegetation and soil patterns in the Marlborough Sounds, South Island, New Zealand. New Zealand Journal of Botany, 24(2), 293-313.
