Shannon catchment includes Broke Inlet and all the wetlands within its catchment, plus the Shannon River and numerous small lakes and swamps on the plains around the Inlet, e.g. "East Broke Inlet Lake", "Deeside Lake". Most of the information here relates to this southern part of the catchment.
Landform: Macroscale irregular estuary (inlet), including six microscale islands; leptoscale-microscale (width) irregular river; microscale round sumplands; and mesoscale-macroscale irregular floodplain and palusplain. The Shannon River has a small delta where it enters the inlet. There are some low-lying wetlands, with soils of sand and clay, and a coastal dune system up to nine kilometres wide, encroaching into the wetlands.
Geological setting: The Shannon River originates in a lateritic plateau (edge of the Yilgarn Craton) and drains through granite/gneiss upland (Albany-Fraser Orogen) to a transitional lowland zone, in which Broke Inlet and associated swamps lie, before breaking through massive consolidated dune-ridges and cliffs (to more than 100 metres) to the ocean. Swamp soils are iron podsols and solonetzic soils with shallow A horizons and there are deposits of peat up to one metre deep in the beds of some small lakes.
Climate: This catchment is in the highest rainfall area of the south west. Median and mean annual rainfall at Walpole are 1325 millimetres and 1346 mm respectively (compared to 900 mm at the Shannon headwaters), decreasing inland. Rainfall is mainly in April-October, peaking in June and July, but there are also occasional summer storms. Annual evaporation is about 1200 mm. In the last 30 years rainfall near the coast has decreased by about 200 mm.
Water supply: The Shannon River originates in swampy flats (e.g. Gobblecannup Swamp) about 40 kilometres north of Broke Inlet. At least ten other streams flow directly into Broke Inlet, notably Forth River and Inlet River, and its catchment extends five to nine kilometres in all directions. The total catchment was slightly disturbed in the past (Shannon basin 15% logged) but apart from small areas in the north-west is now totally protected. The inlet is connected to the sea by a 3.5 kilometre channel, 250 metres wide; a sand bar at the ocean is open during June-September in most years
Inundation: The inlet and lower reaches of the rivers are permanent; other wetlands seasonal, dry for probably one to two months in autumn. When the bar is open, lower reaches of the Shannon River (for over three kilometres), Forth River (300 metres) and Inlet River (two kilometres) may be tidal; fluctuation in the inlet from once-daily tides is less than ten centimetres due to the narrow ocean connection.
Water depth: Broke Inlet is five to six metres deep in the outlet channel, otherwise up to three to four metres deep; fluctuations of up to approximately three metres in summer-autumn create three sub-basins, separated by sand. Floodplain lakes, ponds and swamps (e.g. East Broke Inlet Lake) are 1-1.5 metres deep. Shannon River is three to ten metres deep in its lower reaches.
Water salinity: Rivers, streams, ponds and swamps are fresh, poikilohaline (e.g. East Broke Inlet Lake 0.14 ppt in winter, 1.11 ppt in autumn); inlet waters vary from 5-30 (exceptionally 35) ppt, but are usually hyposaline.
Water pH: The small lakes are acidic: East Broke Inlet Lake 4.5, Deeside Lake 4.8.
Water colour: Small lakes: brown to black.
Ecological role: The network of freshwater wetlands within the site provide all the life-cycle requirements of (probably substantial) populations of three fishes endemic to the bioregion: Black-stripe Minnow Galaxiella nigrostriata, Salamanderfish Lepidogalaxias salamandroides and Balston's Pygmy Perch Nannatherina balstoni. The inlet is a significant drought refuge area for Musk Duck.
Plant structural formations: Low open/closed-forest occurs along streams and in the margins of ponds and swamps; open/closed-heathland occurs extensively and sedgeland in places, in the floodplain wetlands. Dryland vegetation within the basin varies from open-forest to open-heathland.
An outstanding example of an unspoilt entire catchment (freshwater river and estuary/inlet system with associated floodplain), in south-western Australia.
Threatened Species: The gazetted rare Kennedia glabrata.
Composition: The wetland forests are dominated by cedar Agonis juniperina and Banksia, Eucalyptus and Melaleuca species; the wetland heathlands by A. floribunda and Beaufortia sparsa; and the sedgelands by Baumea and Leptocarpus species. There is a narrow fringe of the sedge Juncus kraussii and paperbark Melaleuca cuticularis to much of the inlet, especially the south-west. At least 30 wetland orchid species occur in the parks and presumably many of these within the site. The aquatics Myriophyllum tillaeoides and Triglochin procera occur in some lakes. Potentially rare or restricted (in distribution) Adenanthos detmoldii, Lomandra ordii, Reedia spathacea and Restis jacksonii occur in the floodplains/palusplains of this site.
Notable fauna (waterbirds):
Threatened Species: None.
Composition: 18 species recorded, three listed under treaties.
Numbers: The highest number of waterbirds counted was 2505 in April 1988. The most abundant species are Black Swan (963) and Musk Duck (635, regional rank 2). The site is also regionally significant for Little Pied Cormorant (330, regional rank 5) and Great Crested Grebe (55, rank 3).
Notable fauna (other taxa):
Threatened Species: None.
Composition: The inlet supports 21 species of fish, 16 of them commercial fishes; non-commercial species include Pouched Lamprey Geotria australis, Beaked Salmon Gonorhynchus greyi and Long-finned Goby Favonigobius lateralis. The site is probably significant for river migration of pouched lamprey since the river is fresh and lacks dams or other artificial barriers (salinisation and barriers are obstacles to lamprey migration and breeding in many other southwest rivers). Five species of freshwater fish were recorded at East Broke Inlet Lake in autumn 1992, including two species endemic to the far south coast of the State: Black-stripe Minnow and Balston's Pygmy Perch. Salamanderfish is known from several other small wetlands in the system and probably is plentiful on the floodplains. Marron and koonacs and tortoises occur in the freshwater wetlands. The benthic fauna of the inlet numbers 22 taxa and eight foraminifera taxa have been recorded.
Social and cultural values:
Economic: A small scale fishery in the inlet has yielded 5-10 tonnes of fish annually since 1965, mainly Yelloweye Mullet, also King George Whiting. Recruitment of commercial species is from the ocean so the success of catches depends on the duration of bar opening.
Recreation: The site is popular for bush walking, canoeing (Shannon below Chesapeake Road is navigable), nature study, recreational fishing, horse riding and touring and recreational use is likely to increase. There are interpretation and camping facilities at Shannon townsite.
Aesthetics: The site has high aesthetic value: Near-wilderness landscapes, unspoilt wetlands of several types, vastness and easy access to several parts. The Shannon River is classified "wild"; few such rivers occur in south-western Australia.
Land tenure and land use
On site: Almost entirely within Shannon (National) Park and D'Entrecasteaux National Park; the waters of the inlet are surrounded by the latter park and are classified marine. A very small area of land on the inlet shore at Camfield is a shire reserve.
Surrounding area: National Park, reserve, State Forest and freehold.
Current land use
On site: Nature conservation, recreation and small commercial fishery. Surrounding area: Nature conservation, recreation, forestry, some pasture grazing and low human population. Most of the site has a buffer of native vegetation several kilometres wide. Shacks are established in the shire reserve at Camfield. Much of the upper Gardner River is in alienated land, half cleared for small farms.
There is a small group of fishermen's cottages near Broke Inlet, and the inlet attracts a number of recreational fishermen as well as supporting a small commercial fishery.
Disturbances or threats
Current: Exotic fishes (trout, which have been released in the Shannon River).
Potential: Possible future water supply demands, mineral sand exploration and mining.
Conservation measures taken
WADCALM Management Plan (No. 6) covers the parks. Part of the site (D'Entrecasteaux National Park) is included on the Register of the National Estate.
Management authority and jurisdiction
Parks are managed by WADCALM for NPNCA. Their district office is located at Walpole.