The assessment of shear stress and bed stability in stream ecology


Arved C. Schwendel, Ecology Group, Institute of Natural Resources, Massey University, Private Bag 11222, Palmerston North 4442, New Zealand. E-mail:


1. Substratum stability and shear stress exerted by flowing water can have a strong influence on the structure of benthic communities. Bed stability can be characterised in a variety of ways, e.g. flow competence, threshold of particle entrainment, measures of erosion and deposition, particle transport distance, abrasion and bedload transport rate. This paper reviews methods for the quantification of bed stability and shear stress in streams and rivers that are relevant for the examination of the relationships between stream biota and bed stability.

2. The most suitable method for a research project depends mainly on the objectives. The targeted group of biota, spatial and temporal scale of investigation, as well as hydraulic conditions and substratum characteristics at the study site(s) determine the choice of a technique for the assessment of bed stability.

3. Indirect measurement of shear stress can be more accurate than calculations based on the DuBoys equation. However, the latter is preferred for reach-wide applications within the limits imposed by hydraulic conditions. The entrainment of the substratum is most effectively assessed using a combination of shear stress and competence equations, but the latter require careful parameterisation. At the patch-scale, direct measurement of entrainment force is a valid alternative.

4. Morphometric budgeting is the most comprehensive and least invasive technique for the assessment of rates of erosion and deposition. The transport of substratum particles is efficiently monitored with in situ marked or active tracer particles which allow for rapid and non-invasive identification and high recovery rate. As the assessment of bedload transport rate by formulae can be inaccurate, direct measurement is preferred. However, bedload traps interfere with the substratum and continuity of measurement with samplers is limited. Thus developments in the sector of acoustic and piezoelectric devices offer a potential alternative.

5. The abrasive forces by suspended sediments on stream biota are effectively evaluated with artificial blocks that are fixed on the stream bed. Descriptive surveys that assess bed stability offer an alternative to direct measurement and calculations. They are straightforward and non-invasive but can be observer-biased. If single methods do not provide useful links with biological data this may be improved by the application of a multivariate approach.

6. Many of the methods assessed have not yet been applied in research on benthic communities, but these hydraulic and geomorphologic techniques offer considerable potential for the assessment of bed stability in stream ecology.