Yeagarup Dunes in D’Entrecasteaux National Park is a mobile system of aeolian (wind blown) dunes currently moving inland at a rate of 4 metres a year.
This coastline experiences an annual cycle of sand being removed from the beaches over the stormy autumn to spring seasons during big swells and a slow, gradual beach building with sand being redeposited over the summer months.
During the last Ice Age, which occurred from approximately 25,000 to 8,000 years ago, the northern hemisphere was completely encased in ice. The southern hemisphere wasn’t so badly affected but the climate became much drier and windier than today.
Along the D’Entrecasteaux National Park coastline there was an extra 30 kilometres of land as the sea level had receded dramatically with water being taken up in ice in the northern hemisphere. Throughout the southern hemisphere, sand was deposited along the new coastline and the sand was piled up into massive dune systems by the winds.
When the Ice Age ended, the sea level rose and submerged much of the coastline. The Yeagarup Dune system was too large to be swallowed by the ocean and over the past 12,000 – 8,000 years has been blown further inland by the winds.
The prevailing winds through the area are generally south-westerly and are moving the dunes in a north-easterly direction. The high rainfall of the area (currently around 1100 mm annually) helps to pack the sand down and prevents it from moving more rapidly.
Over Summer/early autumn when the area receives lower rainfall levels the winds tend to be from the north or north-east, pushing the dunes back onto themselves slightly. The sand has a high silica content and the grains pack down tightly, forming high ridges along the top of the parabolic dunes. These ridges will eventually reach a critical point and then fall, creating drifts of soft sand at the base of the dune.
Along the inland face, the dunes are progressing over Jarrah/Marri forests growing on old established, stabilised sand deposits and changing wetlands. Naenup Swamp, which is evident on old historic maps of the area, has almost completely disappeared. A large dune has been approaching Little Yeagarup Lake and has forced the water to push further to the north, doubling the surface area of the lake over the past couple of years.
Yeagarup Dunes is currently 8 kilometres inland from the coast and coastal heathland vegetation has encroached behind it. Throughout the dunes, old Jarrah stumps are being re-exposed after being buried for thousands of years as the sand moves through.
Pemberton Discovery Tours have been visiting the Dunes since 1999 and offer a safe and easy way to enjoy this unique National Park.
Sources: Shannon and D’Entrecasteaux National Parks Management Plan No. 71 2012, Department of Environment and Conservation, Conservation Commission Western Australia. Geology of Western Australia’s National Parks – 2nd Edition, Peter Lane, 2011