A model for accurate drift estimation in streams

1. This paper explores the experimental difficulties involved with the use of drift nets in small streams, and outlines a method whereby the estimation of drift density (number of specimens m^?3 of water) can be improved.
2. Changes in the filtering efficiency of the net caused by trapping of organic debris ('clogging') has the effect of reducing net entrance velocities, causing errors in the calculation of sampled water volume, and thus drift density. A model of the reductions in net entrance velocity based on empirical measurements of trapped debris is developed.
3. Cross-sectional velocity calculations suggest that errors can also be introduced into drift density calculations by positioning sampling nets only on the bed. A method to allow this effect is demonstrated.
4. As adjustments to the calculation of sampled volume are required when sampling in rivers that undergo marked changes in discharge during the sampling period, a method whereby these effects can be accommodated to improve drift density estimations is also outlined.
5. The results of this study imply that theoretical links between flow hydraulics and short-term drift behaviour are poorly understood.