Summer halocline formation was found in a not only non-tidal, but also torrential (winter wet season) middle reach of the Murray River, Western Australia. This phenomenon stems from the reversed longitudinal salinity profile of the river. After saline flow (> 5000 mg/l) from inland agricultural catchments ceased in January, low flows from adjacent perennial freshwater tributaries (< 200 mg/l) overrode the residual saline water in pools; a highly stable halocline and pycnocline at 3.5-4.0 m, formed by the entrainment process, remained through the dry season to April. The interface in one of the long (0.6 km), deep (8.7 m) pools was described by monthly temperature, oxygen, salinity, light transmission (quantum meter) and derived density profiles; reflectance attained a maximum at the interface with the dark saline bottom layer. The copepod Sulcanus conflictus occurred near the surface in January but it was absent later when the upper layers had been diluted below 4000 mg/l, the species' lower tolerance limit. The crayfish Cherax tenuimanus was unaffected but obviously confined to the 42 % of the pool bed above the anoxic zone.