RESUMO
The Australian climate is highly variable and many Australian estuaries lack a seasonal pattern of freshwater flow. During periods of low freshwater inflow, saline waters enter from the ocean through the estuary mouth. These saline waters enter as density currents or as a result of tidal mixing. During periods of high freshwater inflow from the estuary catchment, salt water is flushed towards the estuary mouth. As a consequence, the saline structure of Australian estuaries can be highly variable, depending on the antecedent rainfall. The Richmond River in northern New South Wales in such an estuary. The biota inhabiting estuaries have varying levels of freshwater and saltwater tolerance and reducing the freshwater flow into and along an estuary will favour saltwater species. However, if extractions of freshwater from an estuary are sufficiently high, freshwater habitat could be eliminated entirely for short periods (-one month) but with consequent, and perhaps long-standing, damage to the environment. This contribution describes a new approach to quantify the impact of changes to freshwater flows to the Richmond River estuary. This approach includes a review of hydrological data gathered over the past century and includes a detailed assessment of: changes to the highly variable freshwater inflows and freshwater extraction below the tidal limit; consequent changes to the highly variable saline structure; and the risk to aquatic biota. The crucial factors of magnitude, frequency and duration of short-term intrusions of saline water into freshwater habitat and their impact are quantified by the methods developed.
