Exposed coarse-grained riverine sediments ranging from gravel to cobbles and small boulders.
Species depend on the resorting of sediment during periods of high flow, whose frequency and severity usually peak in the winter away from montane areas. Sparsely vegetated, freshly deposited sediment is exploited for breeding as water levels fall in the spring. Some species can also occur on exposed coastal or lacustrine shingle or in artificially disturbed sites such as gravel pits, but only in the early stages of vegetational succession.
Assemblages on large sedimentary bars in the piedmont zone tend to have a higher species richness than smaller bars on streams in the upper catchment.