PHYSICAL CHARACTERIZATION OF MICROFORM BED CLUSTER REFUGIA IN 12 HEADWATER STREAMS, NEW-ZEALAND

Recent sediment transport research has demonstrated that microform bed clusters (MBC) are particularly resistant to entrainment during flood s and preliminary biological surveys have shown that such structures c ould be providing important refugia for benthos in streams. We therefo re surveyed MBC in a selection of headwater streams, South Island, New Zealand to determine how common such structures are and then related their occurrence to flow variability, relative armouring, reach gradie nt, and sediment geology/geometry to obtain a greater understanding of hydrological and hydraulic stream-scale factors affecting their densi ty and composition. MBC were present in all streams and ranged in dens ity from 0.067 to 0.279 m(-2). They occupied up to 4.4% of the surface area of the survey reaches, generally had 2-3 particles (a maximum of 7), and the average size for the largest particles ranged from 18.5 t o 42.8 cm. MBC density and percentage site cover was significantly rel ated to the relative armouring, but not to flow variability. The numbe r of sediment particles per cluster was significantly correlated with reach gradient, although sediment geology, and associated geometry of particles, also appeared to be important. Thus, density and structure of MBC appeared to be primarily controlled by the interaction of relat ive armouring and reach gradient with highest densities and the larges t number of cluster particles occurring in steep streams with high bed armouring. These structures were sufficiently common to be playing a significant role as flood refugia for benthos in some high gradient, h eadwater streams.