patterns, has reduced the periods of high flow/low salinity
and low flow/high salinity. The periods in which salt
concentrations exceed the critical thresholds of biota now
rarely, if ever, occur, but secondary salinisation, caused by
run-off from the terrestrial landscape, has increased the
amount of salt entering rivers. The reduction in the frequency
of high-flow (flushing) events is causing an accumulation of
salt in these river systems and a gradual increase in the mean
concentration over time (Close 1990; MDBC 1999; DLWC
2000). While the salinity threshold levels for mature biota
may no longer be exceeded, the mean salinity thresholds for
more sensitive life stages may eventually be surpassed
(Fig. 1B).
A similar pattern of salt accumulation occurs in wetlands.
Prior to the removal of the terrestrial vegetation, most of the
water carrying dissolved salts from the surrounding
catchments was trapped by vegetation and transpired or
evaporated. The salt that did wash into wetlands became
concentrated by evaporation, often exceeding tolerance
levels of sensitive biota. Once these wetlands dried the salt
accumulated in the sediments and was removed by flushing
during the next high-flow events. Removal of vegetation has
increased the amount of water entering the groundwater and
the amount of water and salt that enters wetlands. Many