The influence of erosion sources on sediment-related water quality attributes.

Suspended fine sediment has a significant impact on freshwater quality variables such as visual clarity (VC). However, freshwater quality is related to the attributes of the catchment sources contributing fine sediment to the stream network. Here, the extent to which an array of sources defined spatially according to erosion process and geological parent material may be discriminated and classified based on sediment-related water quality (SRWQ) attributes that potentially affect VC was examined. Erosion sources were sampled across two New Zealand catchments representing six types of erosion and eight parent materials. Erosion source measurements focused on particle size, organic matter content, and light beam attenuation (which is convertible to VC). The source data were analysed to: 1) evaluate source variability using a combination of Kruskal-Wallis and principal component analysis; 2) reclassify sources using a Random Forest model; and 3) demonstrate how erosion source affects VC for a range of theoretical sediment concentrations (SC) using a simple empirical model. The results indicate that SRWQ attributes show significant variation across erosion sources. The extent to which attributes differed between sources often related to whether there was a strong association between a specific erosion process and parent material. The 19 a priori source classifications were reduced to 5 distinct sources that combined erosion process and parent material (i.e., bank erosion-alluvium; mass movement-ancient volcanics; mass movement-sedimentary; surficial erosion; gully-unconsolidated sandstone). At low SC, the impact of erosion source on VC became most evident ranging from 2.6 to 5.6 m at SC of 5 g m-3. These findings show how catchment sources of sediment, in addition to sediment concentration, influence VC, and highlight the need to consider quality as well as quantity of material supplied to stream networks when planning erosion control.

Characterization and quantification of suspended sediment sources to the Manawatu River, New Zealand.

Knowledge of sediment movement throughout a catchment environment is essential due to its influence on the character and form of our landscape relating to agricultural productivity and ecological health. Sediment fingerprinting is a well-used tool for evaluating sediment sources within a fluvial catchment but still faces areas of uncertainty for applications to large catchments that have a complex arrangement of sources. Sediment fingerprinting was applied to the Manawatu River Catchment to differentiate 8 geological and geomorphological sources. The source categories were Mudstone, Hill Subsurface, Hill Surface, Channel Bank, Mountain Range, Gravel Terrace, Loess and Limestone. Geochemical analysis was conducted using XRF and LA-ICP-MS. Geochemical concentrations were analysed using Discriminant Function Analysis and sediment un-mixing models. Two mixing models were used in conjunction with GRG non-linear and Evolutionary optimization methods for comparison. Discriminant Function Analysis required 16 variables to correctly classify 92.6% of sediment sources. Geological explanations were achieved for some of the variables selected, although there is a need for mineralogical information to confirm causes for the geochemical signatures. Consistent source estimates were achieved between models with optimization techniques providing globally optimal solutions for sediment quantification. Sediment sources was attributed primarily to Mudstone, ≈38-46%; followed by the Mountain Range, ≈15-18%; Hill Surface, ≈12-16%; Hill Subsurface, ≈9-11%; Loess, ≈9-15%; Gravel Terrace, ≈0-4%; Channel Bank, ≈0-5%; and Limestone, ≈0%. Sediment source apportionment fits with the conceptual understanding of the catchment which has recognized soft sedimentary mudstone to be highly susceptible to erosion. Inference of the processes responsible for sediment generation can be made for processes where there is a clear relationship with the geomorphology, but is problematic for processes which occur within multiple terrains.