The Mitchell River Delta and its Silt Jetties date back some 6000 – 10000 years. When the Silt Jetties were first mapped in 1840 their area was twice the current size. Changes in water salinity, subsequent dieback of shoreline reeds and the removal of vegetation have resulted in wind and water erosion. Had the Silt Jetties been left in a natural state they may have by now been a series of natural islands. The speed of environmental change has increased markedly since European settlement.
River deltas form when a river carrying sediment reaches either a body of standing water, such as a lake, ocean, or reservoir, another river that cannot remove the sediment quickly enough to stop delta formation, or an inland region where the water spreads out and deposits sediments. The tidal currents also cannot be too strong, as sediment would wash out into the water body faster than the river deposits it. Of course, the river must carry enough sediment to layer into deltas over time. The river's velocity decreases rapidly, causing it to deposit the majority, if not all, of its load. This alluvium builds up to form the river delta. When the flow enters the standing water, it is no longer confined to its channel and expands in width. This flow expansion results in a decrease in the flow velocity, which diminishes the ability of the flow to transport sediment. As a result, sediment drops out of the flow and deposits.
There are four main types of deltas classified by the processes that control the build-up of silt. Wave-dominated, tide-dominated, Gilbert deltas, and Estuarine deltas.
In a wave-dominated delta, the movement of waves controls a delta's size and shape. The Nile delta (shaped by waves from the Mediterranean Sea) and Senegal delta (shaped by waves from the Atlantic Ocean) are both wave-dominated deltas.
Tide-dominated deltas usually form in areas with a large tidal range, or area between high tide and low tide. The massive Ganges-Bramahputra delta, in India and Bangladesh, is a tide-dominated delta, shaped by the rise and fall of tides in the Bay of Bengal.
Gilbert deltas are formed as rivers deposit large, coarse sediments. Gilbert deltas are usually confined to rivers emptying into freshwater lakes. They are usually steeper than the normal flat plain of a wave-dominated or tide-dominated delta. This type of delta was first identified by the geologist Grove Karl Gilbert, who described mountain streams feeding ancient Lake Bonneville. (Utah's Great Salt Lake is the only remnant of Lake Bonneville.)
Estuarine deltas form as a river does not empty directly into the ocean, but instead forms an estuary. An estuary is a partly enclosed wetland that features a brackish water (part-saltwater, part-freshwater) habitat. The Yellow River forms an estuary, for instance, as it reaches the Bohai Sea off the coast of northern China.
When published to log this cache you needed to visit the listed coordinates and the two way points and send us the answers to the following questions.
1) Did the Silt Jetties get larger or smaller in area between 1940 and today?
2) What caused “The Cut” in 1919?
3) What type of Delta are the Silt jetties.
However the local Council took the information sign down at WP1, so for now I am putting some pictures in the cache listing to assist you. You will how ever have to show that you were there, so there is an additional requirement to put something in your log to show you were at WP1.
The road out to the end of the final waypoint is dirt and fine for 2WD vehicles, however it is narrow and has some blind corners, so please drive carefully.
More information here: https://www.nationalgeographic.org/encyclopedia/delta/