No abstract

Minimum flows in rivers and streams aim to provide a certain level of protection for the aquatic environment. The level of protection is described by a measure such as a prescribed proportion of historic flows, wetted perimeter or suitable habitat. Conflicting minimum flow assessments from different instream flow methods are arguably the result of different environmental goals and levels of protection. The goals, the way in which levels of protection are specified, and the relationship between levels of protection and the aquatic environment are examined for three major categories of flow assessment methods: historic flow, hydraulic geometry and habitat. Basic conceptual differences are identified. Flow assessments by historic flow and hydraulic methods are related to river size and tend to retain the ‘character’ of a river. Habitat‐based methods make no a priori assumptions about the natural state of the river and flow assessments are based primarily on water depth and velocity requirements. Flow and hydraulic methods assume that lower than natural flows will degrade the stream ecosystem, whereas habitat methods accept the possibility that aspects of the natural ecosystem can be enhanced by other than naturally occurring flows. Application of hydraulic and habitat methods suggests that the environmental response to flow is not linear; the relative change in width and habitat with flow is greater for small rivers than for large. Small rivers are more ‘at risk’ than large rivers and require a higher proportion of the average flow to maintain similar levels of environmental protection. Habitat methods are focused on target species or specific instream uses, and are useful where there are clear management objectives and an understanding of ecosystem requirements. Flow and hydraulic methods are useful in cases where there is a poor understanding of the ecosystem or where a high level of protection for an existing ecosystem is required. © 1997 John Wiley & Sons, Ltd.

(1991) Microhabitat preferences of benthic invertebrates and the development of generalised Deleatidium spp. habitat suitability curves, applied to four New Zealand rivers, New Zealand Journal of Marine and Freshwater Research, 25:2, 187-199, DOI: 10.1080/00288330.1991 Marine and Freshwater Research, 1991, Vol. 25: 187-199 0028-8330/2502- velocities, Potamopyrgus antipodarum and Chironomidae low to moderate velocities (0.0-0.75 m s -1 ), and all three were associated with a broad range of substrates. Deleatidium spp., Olingaferedayi, Hydrobiosidae, and Aphrophila neozelandica were found in a wide range of habitats. Velocity, depth, and substrate suitability curves developed for Deleatidium in each of the four rivers also demonstrated the broad habitat tolerances of this genus. Generalised suitability curves formed by enveloping, rather than averaging, curves from each of the rivers performed favourably when compared to models based on suitability curves developed for each river individually. Correlations between Deleatidium abundance and the joint suitability function, calculated from the generalised velocity, depth, and substrate curves, were significant but poor (r = 0.44-0.69) in each of the four rivers. The intercepts of the linear relationships between Deleatidium abundance and the joint preference factor were not significantly different from zero for three of the four rivers, suggesting that the preference functions are applicable to in-stream flow assessments.

No abstract

The primary components of the physical habitat in a stream are water depth, velocity and substrate size, and these appear to be the best predictors of benthic invertebrate distribution within a stream. Substrate stability and fine sediment deposition also influence benthic invertebrate abundance, with reduced abundance where substrates are frequently disturbed or where fine sediment accumulates. Substrate stability and deposition of fine sediment provide hydraulic constraints on habitat suitability and their limits can be predicted by hydraulic formulae. Hydraulic conditions near or at the surface of the stream bed may have a more direct influence on benthic invertebrates than either water depth or mean velocity in the water column above them. If benthic invertebrates have 'preferred' near-bed conditions, hydraulic considerations suggest that optimum mean water velocities for benthic invertebrates should increase with average stream depth and substrate size. Examination of three sets of field data from rivers of varying size showed that benthic invertebrate abundance was strongly related to mean velocity and substrate size, with large differences in habitat use between species. However, although there were large differences in preferred mean velocities between rivers of different size and depth, there was little evidence that this occurred within rivers of similar size and hence little support for the hypothesis that near-bed hydraulics can provide a generalized method of predicting invertebrate habitat suitability between rivers of different size.