Southwestern Australia’s Estuaries
While in Southwestern Australia, I visited several of their unique estuary systems. Many of these systems routinely close off for months and in some cases years at a time. River-estuary systems in that region of Australia are categorized : permanently open; seasonally open; normally closed; permanently closed. Estuaries regularly open after having been closed for months or years. There are lessons to be learned in Australia, that might be useful in understanding the recovery process for salt marsh restorations in New England.
The best possible source for information on Southwestern Australia’s Estuaries was printed just three weeks before I arrived in Perth. Swanland by Anne Brearly. Published by the University of Western Australia Press. This 550 page compendium was named after the Swan River in Perth and dedicated to Ernest Hodgkins .This book represents the ultimate collection of Southwestern Australian estuary research to date. It was an honor to meet with Anne Brearley and bring a copy of Swanland home to Cape Cod.
Much of the material on this page is sourced from Swanland. I will be sharing many of our restoration planning documents with Australia. We share some of the same characteristics and challenges with estuary restoration. More will be updated on this page.
Map of Southwestern Australia. The Indian Ocean is to the West and the Great Southern Ocean is to the South.
Indian Ocean Estuaries
The Margaret River Estuary is to the right in this photo. In spite of robust sea conditions in the Indian Ocean, this river and estuary system remain cut off from the ocean for several months each year.
A blocked river waiting for rain. Acidic, low oxygen conditions exist during long periods without tidal flushing. The dark water limits penetration of light energy causing thermoclines between warmer surface and cooler bottom temperature water. According to Penn, 1999 and Wilson, 1991 high concentrations of humic (decomposed plant and animal material) organic matter are termed dystrophic. High nutrient supply does not create algae blooms because of the limiting conditions.
This narrow sand spit successfully prevents tidal flushing until seasonal rains elevate river volume.
Water does flow through these naturally diked systems, transported beneath this sand spit by horizontal migration through groundwater. Beneath the freshwater table is a saline lens under this river. Seasonal rains, private and agricultural use of the water table and changing climate all impact water quality and river levels.
Agricultural practices replace multiple canopy layers with a single layer of ground cover. This contributes to increased runoff and reduced infiltration. Agricultural runoff may contain pesticides and fertilizers. Ground water withdrawl for irrigation contributes to increased salinity in the water table.Ground water dynamics impact river levels.
Indigenous forests in this region include multiple canopy layers with 150 foot tall trees. Layered systems, including a mulch layer, contribute to groundwater stability by slowing runoff, increasing infiltration and moderating temperature and evaporation.
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Map of Southwestern Australia showing where indignous vegetation has been lost.
Southwestern Australia’s Great Southern Ocean Estuaries
Wilson River Inlet, near the town of Denmark on the Great Southern Ocean. This inlet is located 1,800 miles from Antarctica and rainfall conditions seem remarkably different. This system also has a weak flow and will often close off in summer but this year it was open. The narrow river mouth reduces 3 foot tides to five inches within the estuary system. Low barometric pressure from Southern Ocean storms control estuary water height more than the tide does. This photo faces a little East of South.
Beach characteristics at the restricted Wilson River Inlet. This photo faces East. The Wilson River is one of the most studied estuary systems in this part of Australia.
Wilson River Estuary vegetation. This photo was taken 1/4 mile inland.
Wilson River Estuary, a few miles inland. The salt tolerant vegetation includes Paper Bark trees. To protect this estuary basin, removal of indigenous vegetation is now prohibited.
When is an estuary not an estuary?
The Cheyne River Inlet on the Southern Ocean. The shallow lagoon nearly dries out but the river usually opens each year. It has been reported open for up to 18 months a few times and closed for 9 years in a row during the 1930′s. The white sand comprising the bar becomes quicksand when wet.