Salt-dependent stratification can occur in freshwater systems following groundwater incursions. Establishment of a salt gradient can reduce mixing and solute transport within aquatic ecosystems. The halocline is a barrier for transport of materials between the surface and bottom strata and has important implications for nutrient and carbon cycling. In particular, it may become a barrier for the movement of oxygen from the surface water to the bottom, causing the rate of oxygen consumption in the bottom waters to exceed the rate of replenishment from the surface, which ultimately leads to anoxia and the death of benthic organisms (Legovic et al . 1991). Anoxia can also alter the microbially mediated cycling of nutrients. Anoxia of bottom waters has been reported in rivers where intrusions of saline water occur (Anderson and Morison 1989; McGuckin 1990; Donnelly et al . 1997; Ryan et al . 1999). Salinity of the groundwater intrusion does not need to be substantially higher than the salinity of the surface water to induce stratification and anoxia. Stratification in the Wimmera River has been observed at a salinity gradient between 300 and 700 mg L –1 (Anderson and Morison 1989). Gribben
et al . (2003) reported the formation of a seasonal salinity gradient in a shallow freshwater wetland of only about 70 mg L –1 , which they attribute to a ground-water intrusion during the drier summer months. This gradient, coupled with a corresponding thermal gradient,z is sufficient to prevent mixing between the surface and bottom waters, with resultant anoxia in the bottom waters.