Variability in the development, persistence and breakdown of thermal, oxygen and salt stratification on regulated rivers of southeastern Australia

Abstract

AbstractDetailed depth profiles of temperature, dissolved oxygen, salinity and oxidation‐reduction potential (ORP) at many sites over two to four years were made on three highly regulated rivers in southeastern Australia which have experienced large‐scale streamflow reductions for urban water supply (Nepean River) or urban, stock and domestic supplies (Glenelg River) or hydropower generation (Snowy River). Well‐developed oxygen stratification was detected on all rivers in summer and was associated with thermal stratification. Hypoxia or anoxia was often measured below the oxycline and reducing conditions (ORP up to −200 mv) were usually associated with such anoxia. Saline groundwater inflows were detected on both the Nepean and Glenelg rivers and produced salt stratification in addition to thermal and oxygen stratification. Density differences of up to 3.7 kg/m3 were determined across the oxycline with no salt stratification, but increased to 14.9 kg/m3 when salt stratification occurred in the upper estuary of the Snowy River. Sediment‐bound phosphorus was released under reducing conditions with order of magnitude increases in total phosphorus concentrations being measured below the oxycline. Similarly, increases of at least 50% and up to one order of magnitude in dissolved iron, aluminium, manganese and/or sulphur were also measured when anoxic and reducing conditions were recorded below the oxycline. Water depth exerted a significant control on thermocline development with depths of at least 5 m, but usually 6 m, being required for thermal stratification in pools. Deep natural scour pools stratify on all three rivers but dredging of weir pools on the Nepean River has increased the extent of stratification. Anoxic and/or hypoxic conditions below the oxycline greatly reduce the amount of aquatic habitat available for aerobic organisms, especially benthic fish. Winds of 50 km/h generated sufficient shear to completely overturn the stratified Cobbitty weir pool on the Nepean River and winds of 28–44 km/h completely mixed epilimnetic waters in Rocklands Reservoir, breaking down temporary thermoclines. Trial environmental streamflows of 21.7 to 40.7 m3/s generated epilimnetic flow velocities that caused sufficient turbulence to effect mixing by gradual entrainment on the Nepean River. However, these flows would need to persist for more than 5 days to break down stratification in the deepest pools. Stratification rapidly redeveloped, even on the recession of the releases. Copyright © 2005 John Wiley & Sons, Ltd.