Swan River estuary

Category: Marine and Oceanic Biomes.

Geographic Location: Western Australia.

Summary: The site of Perth, the capital and largest city of Western Australia, this estuary is the focus of considerable ecological concern because of the cumulative impacts of European settlement.

The Swan River Estuary drains a 49,000-square-mile (126,000-square-kilometer) catchment. Its major tributaries, out of a total of 29, are the Avon River to the north, the Canning to the south, and the Helena to the east. The Avon River contributes the majority of the freshwater flow. The estuary biome is mainly of mediterranean climate, with hot, dry summers and mild, moist winters.

Soils of the Swan coastal plain are generally sandy and low in fertility, with the exception of pockets of more fertile soils around the wetlands that dot the plain and along the river banks. These areas became the focus of early European settlement. The plain has since lost approximately 80 percent of its wetlands to development; the estuary is an urban wetland.

Flora and Fauna

The original vegetation that once fringed the Swan River’s waters was unique and diverse, and included samphire flats (Milyu is the Aboriginal name for samphire), Juncus kraussii sedgelands, forests of paperbark (Melaleuca rhaphiophylla) and flooded gum (Eucalyptus rudis), and stands of swamp oak (Casuarina obesa). It provided habitat for many species of waterbirds and land birds, including migratory wading birds. This vegetation also protected the estuary margins, reducing erosion and filtering nutrients and pollutants flowing into the river.

These areas now encompass mudflats, seagrass beds, and intertidal vegetation. Although little of it remains, natural vegetation along the margins of the Swan River Estuary biome provides many different habitats for a host of animals. Plant species vary significantly with soil-types and other physical considerations. Vegetation in the preserved area includes saltmarshes, samphire flats, sedge-banks, and areas of woodland and shrubland.

Principal among upper-story species is Eucalyptus rudis; while Melaleuca rhaphiophylla and Melaleuca cuticularis with Casuarina obesa form a middle-story, and Juncus kraussii is the main species of sedge. The Swan Estuary suffers extensively from weed infestations, such as black flag (Ferraria crispa) in Milyu; Brazilian pepper tree (Schinus terebinthifolius), bulrush (Typha orientalis), and kikuyu grass (Pennisetum clandestinum) in Alfred Cove and Victorian tea tree (Leptospermum laevigatum) in Pelican Point.

Perhaps the most important wildlife in the estuary are the birds, the most significant of these being the migratory wading birds. Among these are the tiny red-necked stint, pelican, swan, ibis, egret, red-capped robin, white fronted chat, pallid cuckoo, sacred kingfisher, and osprey. There is a small population of variegated fairy wren. There are 33 wading bird species that have been identified here. The migratory species come from as far as Mongolia and Arctic Siberia.

The fauna in Swan River Estuary has not yet been extensively surveyed. The mammals here are mainly small; southern brown bandicoot (Isoodon obesulus) and common brushtail possum (Trichosurus vulpecula) may still be present, but fox, rabbit, and feral cat populations seem to have contributed to the demise of these species.

River dolphin use the adjacent waters, and a variety of fish species—including the yellow-eye mullet (Aldrichella forsteri), Perth herring (Nematalosa vlaminghi), and cobbler (Cnidoglanis macrocephalis)—and prawns use the shallows as both a juvenile nursery and feeding ground. Other aquatic fauna include mollusks, jellyfish, polychaetes (marine annelid worms), and crustaceans.

Human Impact

The Swan River Estuary is part of the customary lands of the Nyungah people, which provided them with both physical and spiritual sustenance. While the traditional inhabitants actively managed the river and its catchment, they did not make substantial physical changes to its hydrology or morphology. This ecoregion became the focus of early European settlement beginning in the early 1600s; initial growth was slow, but the importation of convict labour in the 1850s saw the construction of bridges, drainage and land reclamation, and changes to streambank morphology. Of particular note was the commencement of reclamation of marsh and intertidal land directly in front of the city of Perth.

The next major leap came with the gold boom, which led to two major hydrological alterations in the estuary. The first was the creation of Fremantle Harbor. The estuary had been blocked by a bar at the mouth, and there were extensive mudflats further upstream. The bar was removed in 1896; this also had the effect of increasing salinity in the lower reaches. The second was the construction of Mundaring Weir and the associated water pipeline to Kalgoorlie, which exported quantities of the Swan-Canning catchment’s water to the desert.

The advent of the railway and automobile decreased the significance of the river for transport, while at the same time increasing its popularity for recreation, including fishing. The river’s banks were also used for industry and for waste disposal. For example, the East Perth power station drew cooling water for its operations, and the Attadale foreshore area became a filled-in rubbish dump. A decline in bird numbers was noted in the late 1890s. At the same time, recreational fishing began to be regulated—but it would take until the mid-1950s for an understanding of the ecological role of river shallows to emerge.

Community concern over the health of the estuary has been steadily growing, leading to the proclamation of the Swan River Conservation Act in 1958, and the establishment of the Swan River Conservation Board, which later became the Swan River Trust. The formation of an Environmental Protection Agency in the United States in 1970 corresponded with the establishment of similar organizations in Australia. In the 1980s, the river’s indigenous significance hit the headlines with an ongoing dispute over the proposed redevelopment of the Swan Brewery site along Mount’s Bay Road.

During the 1990s, algal blooms began to become more noticeable in the estuary. In January 2000, a severe bloom forced the closure of the river to all fishing and recreational uses for 12 days. The Swann River was also affected by two significant algal blooms in 2019 and 2020. Public concern lead the government to establish the Western Australia Estuarine Research Foundation in 1994 to set baseline data on water quality and on different species for management and decision-making. The Swan Estuary Marine Park, established in 1999, protects three biologically important areas of Perth’s Swan River: Alfred Cove, 494 acres (200 hectares) adjacent to Melville; Pelican Point, a 111-acre (45-hectare) area in Crawley; and Milyu, 235 acres (95 hectares) adjacent to the Como foreshore in south Perth.

Western Australia’s marine areas are globally significant; its coastal waters are considered to be among the world’s least disturbed. Climate change predictions suggest that all of southwestern Western Australia, however, will become hotter and drier, which has substantial implications for Perth water supply, and for the ecological health of the estuary. Imbalances in this biome could readily impact the offshore ecology, as well as spread inland.

Bibliography

Department of Conservation and Land Management. Management Plan: Swan Estuary Marine Park and Adjacent Nature Reserves: 1999–2009. Perth, Australia: Marine Parks and Reserves Authority and National Parks and Nature Conservation Authority, 1999.

Graham-Taylor, Sue, “A Missing History: Towards a River History of the Swan.” Studies in Western Australian History 27 (2011).

Hamilton, David P. and Jeffrey V. Turner. “Integrating Research and Management for an Urban Estuarine System: The Swan-Canning Estuary, Western Australia.” Hydrological Processes 15 (2001).

Taylor, William. “Rivers Too Cross: River Beautification and Settlement in Western Australia.” National Identities 5, no. 1 (2003).

Trayler, Kerry, and Jeff Cosgrove. “Blooming Surprise.” Landscape, March 2021, www.fish.wa.gov.au/Sustainability-and-Environment/Aquatic-Biosecurity/Documents/Blooming%20surprise%20Landscope%20article%20March%202021.pdf. Accessed 31 Aug. 2022.