Stream habitat hydraulics: interannual variability in three reaches of Catamaran Brook, New Brunswick

The hydraulic habitat of 12 sites in a small salmon stream in central New Brunswick was investigated between 1992 and 1995 to determine patterns of habitat (substrate) stability between and within reaches. Stability was evaluated by measuring particle size distribution in replicated erosional and depositional sites in each reach and calculating the proportion of the bed predicted to be in motion at given flood flows. Erosional (riffle) sites in all reaches showed significant differences (ANOVA, p < 0.05) in substrate particle sizes from year to year, movement of embedded sediment samplers, and high predicted bedload mevement, even in small spates. In contrast, depositional sites (flats, some runs) appeared stable, showing no significant year-to-year differences in particle sizes, no movement of embedded samplers, and no increase in predicted bedload movement until high flow. The impact of the flood on the streambed depends heavily on the particle size distribution present during the flood, resulting in different levels of substrate disturbance during equal-magnitude floods in different years. Certain sites (e.g., flats) may be able to serve as hydraulic refugia to stream fauna during some floods. It is clear that year-to-year variations in substrate stability must be considered when evaluating habitat stability for stream fauna.