Substrate characterisation
There is little quantitative information on sediment composition in the Motueka River, or on trends in sediment composition. An increase in fine sediment has been implicated as a causal factor for changes in the trout population of the river. In order to assess the relationship between trout numbers and sediment characteristics, there is a need is to implement a set of simple, but reproducible, methods for characterising spatial and temporal patterns of bed sediment (or substrate) composition at the reach and habitat level. Ideally, the methods would:
- complement those used during routine drift dive assessments of trout numbers by Fish & Game in the Motueka River and its tributaries (for which information is required for ten 1-km long reaches of the river), and
- be used to identify slugs of fine sediment passing down the river, and determine their origin.
The Motueka River encompasses a wide range of channel form and water depth, and methods for sediment characterisation need to cover this range. A related issue is to provide some justification for the selection of drift dive reaches and the degree to which they represent the range of river and habitat types in the Motueka River catchment. A review report describes the following:
- river and stream classification systems within a hierarchy of spatial scales;
- stream reach assessment methodologies, focusing on methods for substrate characterisation suitable for
- determining the proportion of fine sediment in the river bed substrate,
- use in both wadeable and non-wadeable reaches of the river;
- quantitatively measuring spatial patterns and temporal change.
Most of the techniques reviewed are best suited to streams and rivers smaller than the main stem of the Motueka River and its major tributaries, because they are often only partially-wadeable.
Substrate characterisation of the catchment's rivers is helping set a base line against which future impacts can be measured as well as providing a spatial picture of the extent of fine sediments in the beds of rivers.
Results
Between November and April, annual surveys using a semi-quantitative survey of fine sediment abundance in the Motueka River under base flow conditions are carried out.
At each site the proportion of fine sediment on the river bed was assessed, using class intervals of <1%, 1–5%, 5–10%, 10–20%, 20–50%, and >50%.
Fine sediment was very low at most sites with c.75% of sample points over all the sites having <5% fine sediment and only 6% having >20%.
Note the small number of streams that had greatly elevated amounts of fine sediment compared to this dominant pattern (particularly Herring Stream and Rocky River, and to a lesser extent the Sherry and Little Pokororo rivers). At these sites the fine sediment was dominated by coarse sand derived from Separation Point granite.
Substrate survey results November 2007-08
Substrate assessment at Woodstock drift dive reach, 2009