How variability indices were determined for 130 sites on New Zealand rivers and the sites were divided into groups based on these indices. Univariate and discriminant analyses were used to identify the catchment characteristics which contributed to flow variability. Climate, as determined by topography, geographic location, and the composition of the regolith (especially water storage capacity and transmissivity characteristics), accounted for a broad regional distribution of groups. Flow variability decreased with catchment size and area of lake and, to a lesser degree, with catchment slope. Relationships were found between flow variability, and morphological and hydraulic characteristics. The longitudinal variability of water depth and velocity increased with flow variability, indicating a more pronounced pool/riffle structure in rivers with high flow variability. Mean water velocity at mean annual low, median, and mean flow was higher in rivers of low flow variability than in rivers of high flow variability. There were strong associations with periphyton communities and trout distribution and abundance and a weak association with benthic invertebrate communities. Water velocity was the most important hydraulic variable; it could be linked to changes in water temperature, benthic invertebrate and periphyton community structure, and trout distribution and abundance.