The main physical features of the Geographe Bay catchment are the escarpment backdrop (and source of catchment headwaters) formed by the Darling and Whicher Ranges and the Leeuwin-Naturaliste Ridge overlooking the southern Swan Coastal Plain; wetlands and watercourses; and the shoreline and bay, between Rocky Point and the Five Mile Brook Diversion.
The Geographe Bay catchment is part of the Swan Coastal Plain and Darling Upland landscape character types. The Swan Coastal Plain is a strip of land on the lower western coastline of the State. It consists of three distinctive landform types; foothills, alluvial plains and successive coastal dune systems (CALM, 1994). The Darling Uplands is bordered on the west by Darling Scarp. The land is undulating and dissected with pale, orange laterite soils and gravels (CALM, 1994).
The catchment’s headwaters occur within the Darling and Whicher Ranges and, to a lesser extent, within the Leeuwin-Naturaliste Ridge. The three main sub-catchments are:
- Capel – which incorporates Gynudup Creek and Five Mile Brook as well as the Capel River
- Vasse-Wonnerup – which incorporates Vasse, Sabina, Abba and Ludlow Rivers
- Sussex – which incorporates the Carbunup and Buayanyup Rivers; Annie Brook; minor waterways flowing to Toby Inlet; and the minor streams of Cape Naturaliste, including the Jingarmup, Meelup, Dologup, Dandatup and Dugulup Brooks.
Much of the waterways of the Geographe catchment has been severely impacted with 79% of the 1,084 km total stream length within the catchment having been assessed as ‘highly-degraded and heavily impacted on by upstream and adjacent land-use’ (Pen, 1997). The Capel, Ludlow and Sabina Rivers, and the Meelup and Dologup Brooks retain considerable natural value. The Ludlow River is particularly significant from a biodiversity perspective, being home to four of south WA’s eight endemic freshwater fish.
A series of artificial and modified watercourses occur across the coastal plain portion of the catchment and help to maintain agricultural productivity and provide flood protection for urban areas. A large number of these drains discharge into the Vasse and Wonnerup Estuaries, while the Vasse Diversion discharges directly into Geographe Bay. A number of minor drains flow into Toby Inlet at Quindalup, and the Stirling Drain leads into the Capel River.
The Geographe catchment supports a significant number of wetlands on the coastal plain. But as is the case for the entire Swan Coastal Plain (extending from north of Perth to the Leeuwin Naturaliste Ridge), extensive clearing and drainage modification has led to the degradation or loss of many wetlands.
One of the most important wetland areas is the Ramsar-listed Vasse-Wonnerup wetland system which is also recognised as a Regional Priority. Learn more.
Other important wetlands within the catchment include the Bussell Highway Swamp, Ludlow wetlands (including McCarley’s Swamp), Tutunup Road Lake, Ludlow-Abba wetlands, Broadwater floodplain (including Toby’s inlet) and the Naturaliste Lake Wetlands (Pen, 1997).
The seasonally waterlogged flats on ironstone country at the base of the Whicher Range between the Capel and Carbanup rivers are particularly important because they support rare and threatened plant communities.
There are two main groundwater flow systems in the area – the unconfined aquifer in the superficial (near-surface) formations, and the confined system in the underlying formations (confined water is water held within rock layers).
The Leederville formation is locally important in the area, providing potable water supplies for Busselton and Dunsborough. It ranges in thickness from 50 m in the west to about 500 m in the east of Busselton Shire. Beneath that at varying depths are the Cockleshell Gully Formation and Sue Coal Measures in the west and the Yarragadee Formation in the east.
Areas Managed for Conservation
The Geographe catchment encompasses numerous large and small areas managed for conservation, either by the Department of Parks and Wildlife (DPaW formerly DEC) or the Shire of Capel or City of Busselton. Significant sites include:
- northern boundary of the Blackwood State Forest, Boyanup State Forest, Jarrahwood State Forest, Millbrook State Forest, Mullalyup State Forest, North East Margaret River State Forest and Whicher National Park
- Broadwater Nature Reserve
- Capel Nature Reserve
- Coolilup State Forest
- Fish Road Nature Reserve
- Haag Nature Reserve
- Locke Nature Reserve
- Ludlow State Forest
- Ruabon Townsite Nature Reserve
- Sabina Nature Reserve
- Tuart Forest National Park
- Walburra Nature Reserve
- Yelverton National Park
- Meelup Regional Park
- Tom Hutton Reserve (Shire of Capel)
- Whalers Reach Reserve (City of Busselton)
- Big Rock Reserve (City of Busselton)
- Blythe Reserve (City of Busselton)
- Ambergate Reserve (City of Busselton)
- Carbanup Reserve (City of Busselton)
The vegetation of the Swan Coastal Plain varies in its species composition and structure. In the south (within the Geographe Bay catchment area) there are forests of marri (Eucalytpus calophylla) mixed with jarrah (Eucalyptus marginata) and blackbutt (Eucalyptus patens). Closer to rivers and streams flooded gum (E. rudis) mixes with swamp paperbark (Melaleuca rhaphiophylla). Peppermint (Agonis flexuosa) occurs closer to the coast while on the Darling Uplands forests of jarrah dominate in association with marri.
Clearing of land for agricultural purposes has led to the loss of much of the vegetation in the south west of the State including within the Geographe catchment area (Weaving and Batory, 1998). It has been estimated that only 746 km2 or 37% of the catchment area remains under native vegetation. Approximately 513 km2 or 69% of this remnant vegetation is retained on public land (Connell, Franke, Alder and Jennings, 2000).
Many of the plant communities in the catchment are now considered ‘poorly represented’, i.e. there is less than 30% remaining of the original pre-European extent. The Abba system is particularly important as less than 10% remains; it does not extend beyond the Geographe catchment; and almost all of its remnant vegetation is situated on private land. A further eleven of the soil landscape systems within the catchment have less than 30% of their original extent of native vegetation remaining.
The flora of Western Australia is unique with a high number of species and a high level of endemism. As a result of extensive clearing and modification of the environment many species have become rare and prone to extinction. There are at least 22 Declared Rare species listed within the catchment and numerous Priority Listed species. As further research is undertaken new species may be found which could be added to these lists.
Threatened Ecological Communities
Five threatened ecological communities occur within the catchment. They are:
- Calothamnus granticus heaths on south west coastal granites (Meelup granites) (State listed as Vulnerable);
- Corymbia calophylla woodlands on heavy soils of the Swan Coastal Plain (SCP1b) (State listed as Vulnerable);
- Southern wet shrublands of the Swan Coastal Plain (SCP2) (State listed as Endangered);
- Shrublands on dry clay flats (SCP10a) (a part of the Clay pans on the Swan Coastal Plain TEC – Commonwealth listed as Critically Endangered)
- Shrublands on southern Swan Coastal Plain ironstones (SCP10b) (Commonwealth listed as Endangered).
More information on the Commonwealth listed communities can be found on the Glimpses into disappearing landscapes website here. Copies of the book “Glimpses into disappearing landscapes. Nationally Listed Threatened Ecological Communities of the South West Region” are also available through SWCC.
The fauna of the catchment is a significant environmental resource. Christensen et al. (1985) list 16 marsupial species, 10 mammal species, 7 introduced mammal species, 12 amphibians and 44 reptile species likely to occur in the region. Less is known of the invertebrate fauna but it appears to be extremely diverse (Bradby, 1997).
The fauna of the area is under pressure from several factors including loss and fragmentation of habitat, decline of habitat quality through grazing and land degradation, the introduction of domestic and feral animals and changes to fire regimes. As a result of these pressures population numbers of faunal species have reduced and several species have become extinct. Among those that have been afforded special protection status are: the Western Ringtail Possum, Quenda, Brush-tailed Phascogale, Chuditch, the Rakali or Water Rat, Baudin’s black Cockatoo, Carnaby’s Black Cockatoo and the Forest Red-tailed Black Cockatoo.
Recovery Plans are in place for the Chuditch, Carnaby’s Black Cockatoo, and an Interim Recovery plan is in place for the Western Ringtail Possum. The Western Ringtail possum is of particular significance in the catchment as the species is locally abundant in the Busselton to Dunsborough area and yet this represents one of the last remaining viable populations of the species. The possum’s primary habitat of peppermint woodland is under increasing threat from urban expansion.
Geographe Bay is a north-facing embayment that marks the southern end of the Swan Coastal Plain. Read more.
The dominant landform on the Geographe Bay coast is the Quindalup Dune System. The dunes here are low (1-2m), located on a soft sandy and predominantly north facing coastline. A small area of gneiss rocky coastline also occurs at Cape Naturaliste with small sheltered sandy beaches (at Meelup Regional Park).
While mainly a low energy area, the Geographe Bay coast is regularly affected by north westerly storm exposure and long-shore sediment transport also occurs from west to east.
There is extensive coastal development along much of the Geographe Bay coastline. The remaining areas of coastal reserve are important to retain from a coastal stability and community amenity perspective. The vegetation that occurs in these areas also provides important linking habitat for the endangered Western Ringtail Possum and other conservation dependent fauna such as the Quenda (Southern Brown Bandicoot).
- Geographe Catchment Management Strategy 2008. Geographe Catchment Council