The area of the Barron estuary reporting zone can be defined by the pre-cleared extent of mangrove and saltmarsh remnant vegetation which is characterised by species that require periodic inundation of sea water. This area extends approximately 10.5 km north and 4.5 km south of the river mouth along the coastline and extends approximately 5.5 km inland. The area includes several tributaries that drain into the Barron River and coastal streams that drain directly into the sea.
The assessment area of riparian extent extends further up the river than the mangrove and salt marsh communities to the upper tidal limit and includes vegetation types that are more typical of freshwater environments.
Land use within and adjacent to the Barron estuary area is dominated by urban development. The northern beach suburbs of Machans Beach, Yorkeys Knob and Holloways beach, and the Cairns suburbs of Aeroglen, Cairns North and Whitfield overlay the estuary area.
Industries within the estuary area include the Cairns international and domestic airport, quarries, aquaculture and sugarcane.
The Barron estuary grade has remained ‘moderate’ in 2018-19 with the year on year score decreasing slightly from 54 to 53.
The diagram below shows the detailed results for each indicator by year. Click on the timeslider to see data from previous years.
The Barron estuary water quality remained ‘good’ with the score decreasing from 66 in the previous reporting period to 61. The lowest scoring indicator was chlorophyll a (37) and the grade declined from ‘moderate’ to ‘poor’, with monthly median values frequently not meeting guideline values.
Nutrients (dissolved inorganic nitrogen and filterable phosphorus) remained ‘moderate’ with monthly median values often not meeting guideline values. Dissolved oxygen and turbidity both declined from ‘very good’ to ‘good’ with monthly median values often meeting guideline values.
Pesticides remained “very good” and scored 90, equating to over 99 percent of species protected for the risk assessment metric.
Water quality grades are based on aquatic ecosystem guidelines for protection of estuarine waters and are not based on load reduction targets for the marine environment. Except for pesticides the results were derived from a transect of five monitoring sites in the channel of the Barron River. Pesticides were monitored on the Barron River at the same Great Barrier Reef Catchment Loads Monitoring Program site as used for the basin assessment.
Habitat and hydrology
The habitat and hydrology index is comprised of flow, which is assessed annually, and three longer-term indicator categories that are updated every four years: mangrove and saltmarsh extent, riparian extent (both updated for 2018-19) and fish barriers (update due for 2019-20). Habitat and hydrology remained ‘moderate’ and the score increased from 43 in 2017-18 to 45.
The mangrove and saltmarsh extent scored 39 (‘poor’) with 31.1% loss from pre-clear to 2017.
Riparian extent is assessed to the upper tidal limit of the river which extends upstream of the estuary area that is characterised by mangroves and saltmarsh. The riparian extent scored 23 (‘poor’) with 48% loss from pre-clear to 2017. There was no change to mangrove and saltmarsh extent and riparian extent between the previous assessment for 2013 and the updated assessment for 2017, however the updated Regional Ecosystem mapping (Version 5) resulted in a 0.3% reduction of the estimated loss of riparian extent from pre-clear which was set at 48.3% for 2013.
These results reflect the high level of development within the estuary area. The low mangrove and saltmarsh extent and riparian extent scores for the Barron are mostly the result of historic extensive clearing activity. More recently, mangrove communities in the Barron estuary have been effectively managed to ensure there has been no major new clearing and to allow for some revegetation.
For estuary fish barriers, the Barron estuary scored 61 (‘good’) and is graded ‘good’ for barrier density, ‘poor’ for percentage of stream length to the first barrier, and has an absence of low passability barriers (graded ‘very good’).
The Barron estuary score for flows increased year on year from 49 to 57 but the grade remained ‘moderate’. Flows were assessed in the Barron River at Myola and in Freshwater Creek. In comparison to modelled pre-development conditions, flow alteration occurred to low flows for both rivers, medium flows for Freshwater Creek and high flows for the Barron.
The high rainfall events that commenced in December 2018 were likely to have reduced the effects of impoundments in both rivers (Tinaroo Falls Dam on the Barron River and Copperlode Dam on Freshwater Creek) during and following the wet season, and increased the higher score compared to previous years. The altered flows that occurred may have affected riffle habitats, macrophyte beds, low flow spawning fish, critical hydraulic habitat, longitudinal connectivity, water quality and fisheries production (Stewart-Koster et al. 2018).
The fish index is currently under development and will be available in future report cards.
Stewart-Koster, B., Bofu Yu, B., Balcombe, S., Kennard, M., Marsh, N. 2018 Development of Report Card flow Indicators for the Mackay-Whitsunday and Wet Tropics regions. Australian Rivers Institute, Griffith University and Truii Pty Ltd. Brisbane.