The greatest influence on the coral reefs of Keppel Bay is the fresh water collected by the Fitzroy River from its massive water catchment and carried into the bay.
The Fitzroy River meanders 480 km eastwards through Australia’s second largest seaward-draining catchment—the Fitzroy Basin. At its mouth lies a 25-km-wide strip of tidal mangroves, mudflats, estuaries and ancient beach ridges.
Even in non-flood years, the estuary discharges 500,000 megalitres of fresh water into Keppel Bay, carrying with it sediments, and organic and inorganic matter from the upper catchment and the mudflats of the Fitzroy Delta [8, 10, 11, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22].
About every 10–20 years, monsoonal rains, driven by La Niña conditions, result in major flooding of the Fitzroy and its tributaries, the Dawson and Mackenzie rivers .
If the south-easterly winds are light when this occurs, a massive floating meniscus of fresh, muddy water can spread out beyond the river mouth to the farthest of the Keppel Islands.
It is these floods that are the main ‘reef-shapers’ in Keppel Bay.
The 2011 flood plume making its way towards the Keppel Islands from the mouth of the Fitzroy.© Peter Williams.