The Peel-Harvey Estuary is a broad, shallow system located 70 kilometres south of Perth in Western Australia. It consists of two basins, the Peel Inlet and the Harvey Estuary (Figure 1). These coastal lagoons are connected by a narrow Channel and there is a single natural outlet to the sea, adjacent to the City of Mandurah. This Channel provides restricted exchange, with the daily tidal range within the estuary only 10% of that in the ocean (Bradby, 1997). Freshwater enters the system via three major rivers; the Serpentine and the Murray into the Peel Inlet and the Harvey into the Harvey Estuary.

The major landuses of the 11,000 km² catchment are: agriculture on the coastal plain, forestry on the uplands and a mixture of wheat farming and grazing in the drier interior (McComb and Lukatelich, 1995). The run-off from this catchment is high in nutrients, and the seasonality of the flow results in 90% of the flow entering the estuary between June and September. This, coupled with poor tidal exchange, has lead to nutrient accumulation in the two basins.

This eutrophication lead to two major problems; excessive macroalgal growth in the Peel Inlet, and regular summer blooms of the toxic cyanobacteria Nodularia in the Harvey Estuary. Accumulations of rotting seaweed fouled the sandy beaches with a black ooze and emitted hydrogen sulfide gas in nauseating proportions (McComb and Lukatelich, 1995). In the Harvey Estuary there were reports of fish and swan deaths (Bradby, 1997).

A three part strategy was developed to address the problems of eutrophication in the Peel-Harvey Estuary (Peel Inlet Management Authority, 1994):
    1. reduce the nutrient run-off from the catchment;
    2. continue harvesting macroalgae as necessary; and
    3. increase flushing to the ocean.

The latter of these was achieved by the construction of the Dawesville Channel, a second outlet from the system to the ocean, which was completed in 1994. The increased flushing provided by the Channel was predicted to eliminate Nodularia blooms, increase dissolved oxygen concentrations, improve water clarity and stabilise salinity (Peel Inlet Management Authority, 1994).

At the opening of the Channel on April 23, 1993 the Premier Richard Court said as a part of his opening speech:
"The problems of the estuary have been built up over many decades and likewise the solutions being implemented may take at least five years before full effects are realised" (Bradby, 1997). Nineteen ninety-nine marks the five year anniversary of the completion of the Dawesville Channel, and this report presents the findings of water quality monitoring over the past decade. The objective of this study is to determine if there have been significant improvements in water quality since the opening of the Dawesville Channel.