Lower Blackwood River and estuary
The Lower Blackwood River is usually defined as the part of the Blackwood River from the town of Nannup, down to where it opens into the Hardy Inlet in Augusta. The lower river is influenced by tidal events, receiving estuarine water as far as 42km upstream from the river mouth.
An area of the Blackwood system considered specifically important for ecological values is located within the Blackwood Conservation Park, 30 kms downstream from Nannup township. The main channel of the tributary in this area is contained within State Forest and probably represents the highest quality riparian zone on the main stem or main tributaries of the entire Blackwood River system (WRC, 1997), Although this section of the river is significant, it still drains a substantial area of cleared agricultural land.
Biological and Physical values
The native vegetation of the Lower Blackwood River is dominated by medium height jarrah-marri forest. In the lower reaches a small area of medium height woodland fringes the Blackwood River, comprising flooded gum (E. rudis) and blackbutt with some bullich, jarrah and marri. Low paperbark woodland (Melaleuca sp.) also lines the river in small patches.
Mean annual discharge from the Blackwood River to the Hardy Inlet is about 940GL. The lower river is influenced by tidal events, receiving estuarine water as far as 42km upstream from the river mouth. The main branch of the Blackwood River has relatively high salinity levels due to land clearing in the upstream catchments. An important issue within the Blackwood catchment, is the relative lower salinities of the tributaries compared to the main stem of the Blackwood River.
Seven species of native fish were identified within the Lower Blackwood River in the 2004 study. These included:
- western minnows (Galaxias occidentalis)
- cobbler (Tandanus bostocki),
- nightfish (Bostockia porosa)
- Balston’s pygmy perch (Nannatherina bolstoni)
- western pygmy perch (Edelia vittata)
- soouth-western goby (Afurcagobius suppositus)
- mud / black-stripe minnows (Galaxiella munda / nigrostriata).
Introduced mosquitofish (Gambusia holbrooki) were also identified in the river.
Macroinvertebrates were also sampled at this time. Both amphipods and isopods were found to be present in the Blackwood River indicating saline conditions at that time. The tributaries (Rushy, Mcleod Creeks, Chapman and Upper Chapman Brooks) were found to be a hotspot of freshwater crayfish diversity containing five of the six species of Cherax species endemic to the south-west. Marron (Cherax cainii), gilgies (Cherax quinquecarinatus) and koonacs (Cherax crassimanus) which were found to be almost totally restricted to tributary sites.
Current Condition and Trends
Based on the 2012/13 assessment, the lower reach of the Blackwood River (first 12 km occurring between the confluence of Chapman Brook and McLeod Creek) was assessed and found to be generally in good condition, with four of the ecological index scores assessed (catchment disturbance, fringing zone, physical form and water quality) being categorised as largely unmodified or slightly modified. This section of the Lower Blackwood River is characterised by conservation/minimal use (43%) and grazing (55%). The reach assessed was covered by 73% fringing vegetation, with an average width of 32 m on each bank.
In 1997, approximately 13% of the entire Blackwood Catchment was found to be adversely affected by secondary salinisation. This is as a result of over-clearing in the upper catchment.. The main Blackwood River was historically fresh but the present salinity levels are expected to double over the next 50 years (CSIRO 2001). It is only in the lower reaches, below Bridgetown, where rainfall is sufficiently high and evaporation relatively low, that the tributaries transmit fresh water.
Land-clearing and agriculture has also contributed to elevated nutrient and sediment levels in the lower blackwood resulting in occasional blooms of blue-green algae. Turbidity is high in the Blackwood with levels ranging from 6 NTU, near the confluence with Rosa Brook, to >25NTU near Nannup. Soil erosion and subsequent pool aggradation (filling with sediment) have exacerbated water quality problems, including increased likelihood of pool anoxia (oxygen depletion).
The Lower Blackwood River is located within the identified Lower Blackwood High Ecological Value Aquatic Ecosystem (HEVAE), a nationally recognised aquatic ecosystem. The HVEAE incorporates the lower reaches of the Blackwood River and tributaries including the Upper Chapman and Chapman Brooks, Mcleod and Rushy Creeks. The Lower Blackwood River area is also the subject of a proposed nomination under the Convention on Wetlands of International Importance (known as the Ramsar convention).
Based on the results of the 2012/13 study, a number of potential threats to aquatic ecological health were identified. Dry conditions were observed at a number of sites in February 2013, with the potential impacts of these conditions noted it the assessment of aquatic biota. For example, fish and crayfish species richness was generally lower in the upper catchments of all systems, most likely due to drying. Given the projected decline in mean annual rainfall and runoff for the area, the presence of permanent water refugia during summer will become increasingly important for aquatic biota.
It was estimated that the Yarragadee aquifer contributes approximately 14% of the baseflows in the lower Blackwood River. The salinity of the Yarragadee aquifer varies between 100-400mg/L, therefore, it is potentially an important source of freshwater to the largely brackish Blackwood River. If water is extracted from the Yarragadee aquifer as has been proposed in recent years, this will most likely have a negative impact on the lower Blackwood estuary’s water quality (2004).
Lower Blackwood River and Climate Change
SWCC Strategic Priority
The Lower Blackwood River is identified within SWCC’s NRM Strategy as a first order priority asset under the Water Resources theme.
SWCC have partnered with landholders along the tributaries of the HAVEA to restore and protect riparian areas on private land. This has involved removing aggressive weeds – predominately blackberry, fencing off riparian areas and revegetating with local species.
- Centre of Excellence in Natural Resource Management (2004). Ecological Water Requirements of the Blackwood River and tributaries – Nannup to Hut Pool. Report CENRM 11/04. Centre of Excellence in Natural Resource Management, the University of Western Australia. February 2005.
- White, G, Storer, T & Kitsios, A 2014, River health assessment in the lower catchment of the Blackwood River, Assessments in the Chapman and Upper Chapman brooks, the McLeod, Rushy and Fisher creeks and the lower Blackwood River using the South West Index of River Condition, Water Science Technical Series, report no. 68, Department of Water, Perth.
Header and thumbnail images supplied by Wendy Wilkins.