Waves NOT River Plumes

Sediments from the rivers and from waves resuspension

Main Points

  • the amount of sediment that reaches reefs in river plumes is only about 1% of that caused by wave-resuspension of mud on the seabed. Even this 1% only occurs on inshore reefs. Effectively no mud reaches the GBR-proper which is a long way from the coast.
  • South-Easterly trade winds cause frequent periods when the inshore reefs become “turbid” – muddy.
  • Cyclones cause huge quantities of mud and sediment to be suspended. They completely “reset” the seabed destroying almost everything living there.
  • Stop blaming farmers.
Figure 1: Wave resuspend the muddy seabed in Cleveland Bay. This mud was deposited over many thousands of years. There are coral reefs (mediocre fringing reefs) on Magnetic Island in the background. Those reefs have always lived in water that is much more turbid than the clearwater of the GBR

The rivers of Queensland, and most of Australia, only flow for a few months of the year. Most of the water volume will be delivered in enormous floods that may last only a few days or weeks (see figure). The two biggest rivers of the GBR, the Burdekin and the Fitzroy, have little or no flow most of the time. This is Australia – most Australian rivers are dry most of the time, something that Europeans and Americans often do not appreciate. Every few years, the drought breaks and these two big rivers go into a massive flood often caused by passage of cyclones (hurricanes) that bring torrential rain. During these floods, soil erosion from pastureland into the rivers ultimately arrives at the coast adjacent to the GBR. Around 15 million tonnes of sediment reach the coast adjacent to the GBR each year (Furnas, 2003)[1].

Figure 2  Burdekin River in the dry season (top) and during a flood (bottom). This river is over one km wide. Both pictures are taken from exactly the same location in 2019. The riverbed is usually dry sand with a small water flow meandering within the channel. Every few years a major flood occurs lasting a few weeks. The river then rapidly reverts to its normal, almost dry, state.

Most of the mud delivered by rivers deposits on the seafloor close to the river mouth within a few kilometres of the shore.[2] This is because mud particles contain a lot of clay which, when reaching saltwater, tends to stick together (flocculate) to form bigger and heavier conglomerations that settle rapidly to the sea bed.

Although the mud settles rapidly near the mouth of the rivers, it does not necessarily remain there for long. It will, instead, be resuspended from the seabed by wave action caused by strong winds. For the GBR, the South-Easterly trade winds cause waves that resuspend the sediment close to shore, for roughly two days per fortnight. The quantity of sediment resuspended during these periods of SE trade winds has yet to be measured properly but is probably a few million tonnes and this occurs every couple of weeks (Larcombe and Ridd, 2015).

The winds creating the waves that suspend the sediment also cause a water current to flow north-westwards along the coast. The combination of the waves and the currents results in a north-westward movement of sediment. This movement stops where the sediment reaches one of the sediment traps formed by the northward facing bays such as Cleveland Bay near Townsville, or Trinity Bay near Cairns. These bays are sheltered from the waves and allow the mud to settle more or less permanently. They have a layer of mud up to 10 metres deep that has been deposited over the last 10 000 years. Because these bays are full of mud, the water is often relatively dirty especially when there are strong trade winds.

Figure 3: Turbid (muddy) water caused by high winds all along the coast near Townsville. Some of the reefs of the GBR are seen top-right. They are outside the turbid zone, but there are many “mediocre fringing reefs” in the turbid zone. Conditions like these occur every few weeks and last days. By contrast river plumes are far less common (roughly 10 times less) and produce much lower concentrations of sediment (roughly10-100 times less) on the frinnging reefs. (See page “can you spot the river plume”)
FIgure 4: The water of reefs of the GBR, a long way from the coast, doesn’t get muddy even when there are large waves, because the waves don’t stir up the seabed in the 50-100m deep water around the reef, and there is almost no mud on the reef itself. During cyclones, however, waves easily stir the deep water.

By far the biggest resuspension events occur during cyclones.  Gagan (1988)[3] estimated that during Cyclone Winifred in 1986, around 150 million tonnes were resuspended. Larcombe and Ridd (2015) estimate that Cyclone Yasi in 2011 resuspended 500 million tonnes of sediment. Cyclones can resuspend a layer of sediment tens of centimetres (one foot) in depth over the path of the cyclone that may be 50 km wide. The quantities of sediment moved are prodigious. Enormous sand dunes on the seabed caused by the combined action of cyclonic waves and currents can be seen with sonar equipment used by marine geologists. These events “reset” the seabed – virtually every living organism on the seabed is destroyed. And this likely recurs every few decades for every part of the GBR lagoon (Larcombe and Ridd, 2015)[4]. It can be seen that the movement of sediment by cyclones dwarfs everything else, especially the effect of sediment in river plumes.

Figure 5: It is not known how much mud cyclone Yasi suspended. It probably exceeded 500 million tonnes, about 20 times the total annual load of every river to the GBR coast. These powerful systems move far larger amounts of sediment that what flows to the coast in rivers.

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[1] Furnas, M., 2003. Catchments and Corals: Terrestrial Runoff to the Great Barrier Reef.

Australian Institute of Marine Science and CRC Reef Research Centre, Townsville

(334 pp.).

[2] Belperio, A.P. (1983). Terrigenous sedimentation in the central Great Barrier Reef lagoon: a model from the Burdekin region. Bureau of Mineral resources Journal of Australian Geology and Geophysics, 8, pp.179–190.

[3] Gagan, M.K., Johnson, D.P. and Carter, R.M. (1988). The Cyclone Winifred Storm Bed, Central Great Barrier Reef Shelf, Australia. SEPM Journal of Sedimentary Research, Vol. 58(5), pp.845–856.

[4] Larcombe and Ridd, 2015. The Sedimentary Geoscience of the Great Barrier Reef Shelf context for Management of Dredge Material. Report to Queensland Port Association,

Brisbane.

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