Keppel Bay – origins

Understanding how the islands, seabed and coastline of the Keppel region were formed can help us understand changes to the living reefs that occur within our own lifetimes.

Millions of years ago, Australia’s Great Dividing Range was formed through a complex process thought to include volcanic activity as a rift occurred between two land masses [1].

At that time, the Fitzroy River originated west of the Great Dividing Range and meandered slowly across the floodplain that stretched eastwards from the range to the coast.

Satellite image showing the massive Fitzroy Basin, the catchment of the Fitzroy River, which drains into Keppel Bay just north of Curtis Island.

Over millions of years, as the range and the upper catchment of the Fitzroy slowly eroded, sediments settled on the floodplain forming a thick bed rich in minerals, clay and nutrients [2, 3].

Caulerpa sertularioides (marine macroalgae) thrives on a thick bed of sediment and nutrients. © D. Brighton.

Keppel Bay – a drowned landscape

After the last ice age, the sea level rose 60 metres, flooding the plains and foothills, before falling to its current level.

The submerged floodplain is what we now know as Keppel Bay. The Keppel Islands are simply the hilltops and high points of this drowned landscape.

The river channel at the bottom of Keppel Bay

Today, the Fitzroy River still meanders across the submerged floodplain that we now call Keppel Bay. This underwater river is known as the Capricorn Channel [4].

From the surface, we cannot see this deep, sediment-filled ancient channel, yet it still heavily influences the conditions in Keppel Bay­ [5, 6, 7, 89].

Bathymetry (water depth) map showing the Capricorn Channel in light yellow, and shallow (red) to deep (green) waters along the Capricorn Coast. (Image: OzCoasts)

The 45-metre-deep bed of sediments at the river mouth in Keppel Bay is a fascinating record of the shifting sea level, the geologically diverse catchment, and the massive exchanges of material that occur between the river, the estuary and the bay.

Not all of the sediments carried by the river enter the bay. During times of low river flow, strong tides can pump fine sediments back up into the estuary [10, 11].

The 2011 flood approaches the islands in Keppel Bay. © Peter Williams.

What is certain, though, is that the amount of sediments is steadily increasing over time while sea level and shorelines remain relatively stable [12]. Over hundreds of years, this increasing amount of sediment is likely to slowly degrade the Keppel reefs.