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The source sediment fingerprinting identified the dominant fine sediment source to be originating from Mudstone terrain with model estimates ranging between 37.8 – 46.6 % (Chapter 4). Mudstone sources are abundant throughout the entire catchment, particularly in the eastern sub-catchments where the soft erodible hill country are identified as erosion prone land (Ausseil and Clark, 2007). Cliff erosion, channel and gully incision, as well as mass movements are all processes evident in the catchment, and are all capable of delivering fine mudstone sediment into the active channel.

The confluence based fingerprinting (Chapter 3) also supports a mudstone-laden suspended sediment load by attributing the Tiraumea, followed by the Upper Manawatu sub-catchment,

as main sources of this sediment. C1-2 represents the confluence of the Tiraumea, Mangatainoka and the Upper Manawatu, and the Tiraumea sub-catchment provided the dominant geochemical signature at this major confluence. The Tiraumea represents the sub- catchment with the highest areal percentage of Mudstone and fits the conceptual premise that soft sedimentary terrain is a significant sediment source. The highest mean suspended sediment concentrations of 198 g m-3 are also experienced in the Tiraumea compared to 126 g m-3 from the Pohangina sub-catchment as the next highest concentration (Basher et al., 2012). However, total sediment yield from Upper Manawatu (5,813 t x 103) is over twice that of Tiraumea (2673 t x 103) due to significantly larger flow originating from the Upper Manawatu sub-catchment (Basher et al., 2012). This indicates major suspended sediment loads originating from the Upper Manawatu should be observed in the discriminant function analysis of C1-2 which is not immediately the case (Chapter 3).

The discrepancy presents several implications for interpreting sediment fingerprinting source estimation. Firstly, it could reveal a failing in the sampling method to capture the inherent spatial variability encountered in this channel, or exposes the influence of prior flow conditions and hysteresis in producing temporal fluxes and sediment deposition which may not reflect the prevailing catchment sediment source. Preferential deposition or transport within the Upper Manawatu due to these prior conditions could mean the geochemical character does not completely denote mudstone sediment to the same degree as it would in the Tiraumea due to the presence of the greywacke sandstone source which feeds into the Upper Manawatu. Remobilization of fine sediment stored in the channel will also influence sediment source discrimination and introduces additional complexity to suspended sediment signals, and as Collins et al. (1998) suggests may have an averaging effect on interpretation. A collection of stored sediment in the channel does not necessarily represent a single temporal component, but likely a mixture sediment sources eroded at different times. Koiter et al.

(2013b) points out the issue of primary and secondary sources and at which point significant change in the properties has occurred that stored sediments should represent a new and unique source. Some researchers have explored this component and indicated the significance of remobilized fine sediment as a major source (e.g. Collins and Walling, 2006, Collins and Walling, 2007).

It is also possible the discrepancy is due to a misalignment between sediment size fractions used for geochemical analysis and those associated with the calculated suspended sediment values. Geochemical analysis required sediment samples to be sieved to a < 63 μm grain size fraction to represent the suspended sediment component. However, suspended sediment

samples taken during a storm event (Chapter 5) resulted in a considerable quantity of discarded sediment following extraction of the < 63 μm grain size fraction indicating a component of the suspended sediment collected during the event was > 63 μm. Furthermore, grain size analysis of the confluence based samples (Chapter 3) showed D50 values of the suspended load ranged from 16- 44 μm which situates the samples in the silt range. Flocculation effects from clay particles into silt-sized aggregates are also a possibility that have not been investigated in this research. An estimated 12 - 16 % of fine sediment originates from the Mountain Range (Chapter 4). The Tararua and Ruahine Range represent active mountain ranges where significant erosion occurs throughout the eastern flanks of the Ruahine Range. The relatively low sediment source estimation suggests that the sediment derived from this area is either coarser and contributes more to the bedload than the suspended sediment load, since this sediment originates from the more resistant greywacke sandstone, or there are (dis)connectivities between sediment sources and the active channel. The Hill Surface and Hill Subsurface sediment components provide similar values to one another. Hill Subsurface accounts for 9 – 11 % while Hill Surface accounts for 12 – 16 % of sediment proportions (Chapter 4). These estimates, as well as the similar geochemical signatures, suggest a relationship between the contributing processes. Shallow translational landslides in the Manawatu transport the soil horizon (Dymond et al., 2006b), which would include both the surface and subsurface components. The subsurface component typically comprises a thicker component of the landslide material which should display greater Hill Subsurface sediment relative to Hill Surface sediment sources. Landslides occurring from the hill country are typically underlain by similar geologies which Crozier (2005) identified as being the leading variable in determining the length of runout but does not indicate the relative proportion of surface and subsurface sediment reaching the channel from a landslide. One study in the Manawatu found that following a large storm event, the majority of the eroded material generated remained in the hillslope system, while an average of 25 % reached the fluvial system (Wright, 2005). This is in agreement with the sediment fingerprinting research, which suggests storage effects are could be influencing the relative proportions of subsurface and surface sediment reaching the active channel. Additionally, higher surface sediment proportions could also reflect the agricultural nature of the catchment, whereby intensification of agricultural practices cause significant disturbance to topsoil more so than subsurface soils, allowing greater erosion of the topsoil material.

The Loess sediment accounted for between 9 – 15 % of the sediment proportion in the catchment. This material tends to sit above the mudstone deposits particularly in the

Pohangina sub-catchment. Channel Bank Sediment was found to account for 0 – 4.3 % of suspended sediment and although the Channel Banks are widespread throughout the catchment, they are primarily found within a depositional floodplain environment and don’t provide large enough exposed surfaces relative to Mudstone or Mountain Range sediment sources. Despite channel erosion being a target of widespread management strategies which involve planting streambanks with little demonstration of effectiveness (Marden, 2011). This research suggests that channel bank sediment is of little importance to total suspended sediment yield in this catchment and although erosion of channel banks is visible, the volume of suspended sediment produced is negligible in comparison to other sources. This is in alignment with conclusions drawn by De Rose and Basher (2011a) on research in the Waipaoa catchment which represents similar hillslope terrain to the Manawatu although admittedly displays much higher hillslope erosion rates. De Rose and Basher (2011a) state the sediment derived from cliff retreat (i.e. the mudstone) far outweigh the sediment originating from the alluvial banks. It would also suggest that resources devoted to management of bank erosion should be kept in perspective and successful bank erosion strategies are not going to have a significant influence on sediment loads. Limestone sediment provided little to no contribution to the suspended sediment of the Manawatu River. Sources of Limestone within the catchment are relatively minor and together with highly soluble carbonates within the limestone means that any limestone components delivered to suspended sediment transport would be rapidly dissolved.