Landscapes of the Lower Blackwood


The Blackwood River extends from the coast and the Blackwood Estuary and Hardy Inlet at Augusta to its upper reaches in the Wheatbelt. In the lower Blackwood Basin, a number of tributaries lead into the Blackwood River including Rushy Creek, Chapman Brook and McLeod Creek. Approximately 70% of the lower Blackwood Basin is within conservation estate. The lower Basin also has the lower Scott River running through its southern boundary along the coast. The Scott River and Blackwood River both feed into Hardy Inlet. The lower Blackwood Basin contains some highly prized natural assets, in particularly its highly valued waterways and iconic biodiversity.

Land

The lower Basin (approximately 15% of the overall Blackwood Basin) drains the predominantly sedimentary Mesozoic rocks of the southern Perth Basin and the Proterozoic crystalline rocks of the Leeuwin Complex. Surficial sediments, primarily Tertiary-Quaternary in age, occur also.

The southern Perth Basin is divided into two major structural units, the Bunbury Trough and the Vasse Shelf. The Bunbury Trough, which lies west of the Darling Scarp, contains Palaeozoic sedimentary rocks which range in age from Permian to Early Cretaceous, down to depths of 8000 metres. The Vasse Shelf which lies to the west of the Bunbury Trough contains up to 3000 metres of sediments. Jurassic and Cretaceous sedimentary rocks commonly subcrop on these structural units and are overlain by Early Tertiary and Quaternary age sediments (De Silva et al., 2000). The Leeuwin Complex bounds the southern Perth Basin to the west (Beatty et al., 2009).

The lower Blackwood Basin is located west of the Darling Scarp and can be divided into three physiographic regions; the Leeuwin-Naturaliste Ridge (lower catchment), the Scott Coastal Plain and the Blackwood Plateau. Together the three are termed the Donnybrook Sunklands.

The Donnybrook Sunklands has low surface elevations and the principal drainage channels are less incised than those in the middle Basin.

Learn more about the Leeuwin Naturaliste Ridge, Scott Coastal Plain and Blackwood Plateau

Leeuwin Naturaliste Ridge (lower catchment)

This is a narrow 0.2 to 0.56 km wide strip of land which runs along the coast between Cape Naturaliste and Cape Leeuwin. It comprises a discontinuous ridge of Tamala Limestone with prominent rounded hills ranging between 160 m and 200 m in height and the underlying Leeuwin Black granite exposed in places.

The Leeuwin-Naturaliste Ridge is included in the Register of the National Estate (Giblett & Webb, 1996) and contains hundreds of caves of significance due to the quality of the formations and the cultural and fossil remains they contain. The coastline west of the ridge comprises steep limestone and granite cliffs interspersed with sandy bays and steep sand dunes between rocky headlands.

Scott Coastal Plain

A low-lying, swampy plain featuring windswept parabolic dunes with limestone and granite headlands. Between the headlands area stretches of exposed beaches which are punctuated by rivers and estuaries.

Soils are basically sandy, generally deep dry sands with winter waterlogged sands near the Scott River. Some sand deposits contain economic concentrations of titanium minerals.

Blackwood Plateau

Gently undulating with a low hilly relief, and has broad depressions with swamps, but is at a lower elevation than the Darling Plateau.

Soils are characterised by laterite, gravels and grey sand on upper areas, sandy yellow soils on slopes and river terraces.

Natural Waterways

The Blackwood River is the major river of the lower Blackwood Basin and a number of important tributaries lead into it including Rushy Creek, Chapman Brook, Upper Chapman Brook and McLeod Creek. In the middle-lower parts of the basin, located downstream of the Albany Highway, the river systems become well defined and incised with good relief and relatively steeper longitudinal gradient. The flows over much of the length of the Blackwood River, the largest river system in southwest Western Australia, are seasonal. Below Nannup however, driven by groundwater discharge, the flow has become almost permanent.

Near the town of Augusta the river discharges into Hardy Inlet which links to the ocean. Although the water quality is now unsuitable for domestic supplies, due to dryland salinity in the upper and middle catchments making it too salty, the Blackwood River from Boyup Brook to the Southern Ocean at Augusta is important for wildlife conservation and is valued for recreation and cultural activities. Learn more about the values of the lower Blackwood River here.

The lower Scott River runs along the southern coastal boundary of the lower Blackwood Basin, feeding into the Hardy Inlet along with the Blackwood River. Learn more about the Hardy Inlet here.

Groundwater

The estimated annual groundwater discharge from the South West Yarragadee aquifer into the Blackwood River about 20 km west of Nannup is between 10 and 20 GL. This groundwater discharge is only a small proportion of winter flows, but a major proportion of the summer flows in the lower Blackwood River. There are also small volumes of groundwater discharge from the Leederville aquifer into the tributaries of the Blackwood River in this area.

Wetlands

The Blackwood River (Lower Reaches) and Tributaries System is identified as being ‘nationally important’ in the Directory of Important Wetlands in Australia due to meeting the criteria of:

  • It is a good example of a wetland type occurring within a biogeographic region in Australia.
  • It is a wetland which is important as the habitat for animal taxa at a vulnerable stage in their life cycles, or provides a refuge when adverse conditions such as drought prevail.
  • The wetland supports 1% or more of the national populations of any native plant or animal taxa.
  • The wetland supports native plant or animal taxa or communities which are considered endangered or vulnerable at the national level.
  • The wetland is of outstanding historical or cultural significance. Read more.

The Gingilup-Jasper wetland system is also recognised as a Wetland of National Significance. This system is located at the south-eastern most corner of the lower Blackwood Basin, on the Scott Coastal Plain and within the D’Entrecasteaux National Park and Gingilup Swamps Nature Reserve 30626. The wetland chain crosses into the Warren Catchments area and includes Lake Jasper which is the largest (440 ha) freshwater lake in South Western Australia.

Biodiversity

Areas Managed for Conservation 

Of the entire Blackwood Basin area, the lower Blackwood Basin has the most vegetation within conservation estate, and the least fragmented landscape. Approximately 69% of the lower catchment is designated State Forest or conservation estate including:

  • Barlee Brook State Forest
  • Blackwood State Forest
  • Bramley National Park
  • Cambray State Forest
  • Chester Nature Reserve
  • D’Entrecasteaux National Park
  • Easter National Park
  • Ellis Creek State Forest
  • Forest Grove National Park
  • Gingilup Swamps Nature Reserve
  • Hilliger National Park
  • Jarrahwood State Forest
  • Leeuwin-Naturaliste National Park
  • Millbrook State Forest
  • Milyeannup National Park
  • Milyeannup State Forest
  • Mullalyup State Forest
  • Nannup State Forest
  • North Donnelly State Forest
  • Pagett Nature Reserve
  • Scott National Park
  • South Blackwood State Forest
  • South East Nannup State Forest
  • Whicher National Park
  • Wiltshire-Butler National Park.

Vegetation

Approximately 70% of the lower Blackwood Basin is covered in native vegetation with the majority of this occurring within State Forest and conservation reserves as mentioned above. Much of the vegetation is continuous across the landscape, with clearing occurring around the Nannup townsite and in the western boundary of the area, either side of the Blackwood River and along the Scott Coastal Plain.

The area can be divided into the Warren bio-region (along the coast of the area) and the southern Jarrah Forest bioregion.

Warren Bio-region

The Warren bio-region is a dissected, undulating country of south-west intrusions of the Yilgarn Craton and western parts of the Albany Orogen with loamy soils supporting Karri forest, laterites supporting Jarrah-Marri forest, leached sandy soils in depressions and plains supporting low Jarrah woodlands and paperbark Agonis flexuosa and Banksia woodlands and heaths.

Southern Jarrah Forest Bio-region

The Jarrah Forest bio-region occurs on the Darling Plateau, with the southern Jarrah Forest occurring south of Collie, where the Darling Plateau broadens and is less well drained.  The forest and woodlands are generally similar to those occurring in the northern jarrah forest, but the understorey reflects the wetter conditions. Rainfall is from 1200 mm in the south-west to 500 mm in the east. Vegetation comprises Jarrah-Marri forest in the west grading to Marri and Wandoo woodlands in the east. There are extensive areas of swamp vegetation in the south-east dominated by Paperbarks and Swamp Yate. The understorey component of the forest and woodlands reflects the wetter nature of this area. The majority of the diversity in the communities occurs on the lower slopes or near granite soils where there are rapid changes in site conditions.

Ecological Linkages

Although much of the lower basin remains vegetated, where clearing has occurred, small remnants of vegetation become important, along with the natural waterways, in acting as stepping stones and linkages between these smaller remnants and the larger conservation reserves. Regional ecological linkages have been identified across the lower Basin and can be learned about here. Local linkages can compliment these regional linkages but are identified at a more local scale.

Flora

Even though the lower Blackwood Basin has not nearly been as extensively cleared as the middle and upper basin, several declared threatened flora species have been recorded. These include:

  • Verticordia plumosa var. vassensis (EN)
  • Reedia spathacea (EN)
  • Myriophyllum trifidum (EN)
  • Lambertia orbifolia subsp. Scott River Plains(L.W.Sage 684) (EN)
  • Kennedia lateritia (EN)
  • Grevillea brachystylis subsp. australis (EN)
  • Drakaea micrantha (EN)
  • Daviesia elongata subsp. elongata (VU)
  • Darwinia ferricola (EN)
  • Caladenia lodgeana (CR)
  • Caladenia harringtoniae (VU)
  • Boronia exilis (EN)
  • Banksia nivea subsp. uliginosa (EN)

Numerous other priority flora species have also been recorded within the area.

Fauna

The lower Blackwood Basin supports a diverse range of fauna species. While some species such as the Western Grey Kangaroo are common, many others are now rarely seen or are restricted in range and have been afforded special protection status (Wildlife Conservation (Specially Protected Fauna) Notice 2014); including:

Schedule 1 – Fauna that is rare or likely to become extinct

  • Woylie, Brush-tailed Bettong (CR)
  • Curlew Sandpiper (VU)
  • Great Knot (VU)
  • Forest Red-tailed Black Cockatoo (VU)
  • Baudin’s Black Cockatoo (EN
  • Carnaby’s Black Cockatoo(EN)
  • Great Sand Plover (VU)
  • Chuditch (VU)
  • White-bellied Frog (CR)
  • Orange-bellied Frog (VU)
  • Eastern Curlew (VU)
  • Brush-tailed Phascogale, Wambenger (VU)
  • Western Ringtail Possum (EN)
  • Quokka (VU)

Priority Fauna (DPaW 2014)

  • Quenda (P5)
  • Masked Owl (southern subsp) (P3)
  • Australian Bustard (P4)
  • Bush Stone-curlew (P4)
  • Western False Pipistrelle (P4)
  • Water-rat (P4)
  • Western Brush Wallaby (P4)

To find out the definition of the conservation codes click here.

The tributaries to the lower Blackwood River provide important habitat for a number of endemic native freshwater crayfish (marron, gilgie and koonac) and native freshwater fish (Balston’s Pygmy Perch (VU), pouched lamprey (P1), freshwater cobbler, nightfish, western pygmy perch, black-stripe minnow (P3), western mud minnow (VU).  Read more about these important systems: Lower Blackwood River, Lower Scott River, Rushy Creek, Chapman Brook, Upper Chapman Brook and McLeod Creek.

The Hardy Inlet and Blackwood Estuary also provides habitat for many native fish species as well as migratory birds.

Birds covered under International Agreements (JAMBA/CAMBA) that have been recorded within the area include:

  • Common Sandpiper
  • Cattle Egret
  • Eastern Great Egret
  • Ruddy Turnstone
  • Sharp-tailed Sandpiper
  • Sanderling
  • Red-necked Stint
  • Long-toed Stint
  • White-bellied Sea-Eagle
  • Broad-billed Sandpiper
  • Black-tailed Godwit
  • Rainbow Bee-eater
  • Whimbrel
  • Bridled Tern
  • Glossy Ibis
  • Common Green Shank

Threatened Ecological Communities

Three threatened ecological communities occur within the lower basin. They are:

  • Aquatic Root Mat Community Number 1 of Caves of the Leeuwin Naturaliste Ridge (Commonwealth listed as Endangered)
  • Aquatic Root Mat Community Number 4 of Caves of the Leeuwin Naturaliste Ridge (Commonwealth listed as Endangered)
  • Scott River Ironstone Association (Commonwealth listed as Endangered)

and the priority communities:

  • Reedia spathaceaEmpodisma gracillimumSporadanthus rivularis dominated floodplains and paluslopes of the Blackwood Plateau (P1)
  • Low forest B of Melaleuca cuticularis with Banksia occidentalis over Low Scrub B of Acacia saligna and Rhadinothamnus anceps (P1)
  • Dense heath B of Spyridium globulosum, Banksia occidentalis, Olearia axillaris, Melaleuca pauciflora, Pericalymma spongiocaule and Jacksonia horrida with tall open sedges of Ficinia nodosa (P1)
  • Basalt association of the Warren Region (P2).

Glimpses thumbnailMore 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.

Coastal and Marine

The lower Blackwood Basin has a small portion of coastal and marine environment, around and east of the townsite of Augusta. The marine environment around Augusta is the southern end of the Ngari Capes Marine Park. Much of the coastal area to the east of Augusta remains vegetated but is not within conservation estate. This vegetated coastline does provide important landscape links to the D’Entrecasteaux National Park occurring just within the Blackwood Basin and extending into the Warren Catchments area.

A number of off-shore islands also occur along  the  Capes  and  Augusta  coastline  and  are important seabird breeding areas and haul-out areas for New Zealand Fur seals and Australian Sea Lions. Of the 79 seabird species found in these waters, 11 breed here, 44 are visitors and another 24 have only recently been documented.  Five cetaceans occur in these coastal waters and all are listed for protection status with the Marine Conservation Branch.

The Hardy Inlet is a wave-dominated estuarine system with diverse biophysical characteristics.  The estuary mouth is permanently open to the sea and the tidal influence  has  been  measured  55  km  upstream. Modifications to the estuary include a boat channel and the artificial opening of the previously freshwater Swan Lake system, but much of the high natural values and healthy fringing and water life remain.

Source

 

 Header image by Wendy Wilkins.