Mulgrave Basin

The Mulgrave Basin contains the largest population centres in the region.

At the northern end of the basin is the city of Cairns (population in excess of 170,000) and the Trinity Inlet.

The Indigenous Yarrabah community with a population of approximately 3000 people is located on the coast to the east of Cairns.

The Port of Cairns occupies the lower end of Trinity Inlet and provides limited deep-water ship berthing.

The upper catchment is in the relatively undisturbed rainforest environments of the Bellenden Ker Range. A small portion of the upper Mulgrave is on the edge of the developed agricultural area of the Atherton Tableland.

The river floodplains and the lower river valley are dominated by sugarcane where the environment has been significantly modified with the loss of much of the original lowland rainforest.

The Mulgrave Basin grade has remained ‘good’ with the score increasing from 68 to 73. 

The diagram below shows the detailed results for each indicator by year. Click on the timeslider to see data from previous years.

Water quality

The water quality grade for the Mulgrave Basin remained ‘good’ and the score of 69 improved slightly from 66 in 2018-19.

The sediment score, comprised of the total suspended solids (TSS) indicator, scored 72 and remained ‘good’. Monthly median concentrations of TSS often met guideline values.

The grade for nutrients declined from ‘good’ in the previous year to ‘moderate’ and scored 57. The dissolved inorganic nitrogen (DIN) indicator declined from ‘moderate’ to ‘poor’ with monthly median concentrations mostly not meeting guideline values. Filterable reactive phosphorus (FRP) remained ‘good’, with monthly median concentrations often meeting guideline values.

Pesticides for the Mulgrave Basin remained ‘good’ and scored 78, equating to over 98 percent of species protected for the risk assessment metric.

Water quality grades are based on aquatic ecosystem guidelines for protection of freshwater systems and are not based on load reduction targets for the marine environment. The results are derived from the end of catchment monitoring site at Deeral, capturing 59% of the basin.

Habitat and hydrology

The habitat and hydrology index is comprised of four longer-term indicator categories that are updated every four years: wetland extent, (updated for 2017-18), riparian extent (to be updated for 2020-21), instream habitat modification consisting of impoundment length (updated for 2018-19) and fish barrier indicators (in development), and invasive weeds (updated for 2019-20).

The annual scores for the habitat and hydrology index from 2014-15, 2015-16, 2016-17 represent the changes resulting from the addition of indicators and not changes in the indicator scores themselves, whilst the index scores for 2017-18 to 2019-20 include annual updates to the flow indicator as well as updates to the longer term wetland extent, impoundment length and invasive weeds indicators.

The Mulgrave Basin habitat and hydrology index scored 66 for 2019-20 and the grade remained ‘good’.

The habitat modification score of 100 (‘very good’) is comprised of an impoundment length score of 100 (‘very good’) indicating that less than 1% of the total length of the waterways with a stream order of 3 or higher was impounded by artificial structures.

The fish barrier indicator method is still being developed and will be available in future report cards.

Riparian extent for the Mulgrave Basin scored 78 (‘good’) with 6.1% loss from pre-clear extent to 2013. With the basin predominantly rainforest in the upper and midlands, it is expected that the majority of loss is in the lowlands due to development and land use.

Wetland extent scored 33 (‘poor’) with 37.6% loss of palustrine (freshwater) wetlands from pre-clear to 2017. There was a loss of three hectares of palustrine wetlands since 2013.

The invasive weeds indicator scored 43 (‘moderate’) and represents some impacts of aquatic weeds within the Mulgrave freshwater system including hymenachne (Hymenachne amplexicaulis), salvinia (Salvinia molesta), water hyacinth (Eichornia crassipes) and pond apple (Annona glabra).

The Mulgrave Basin grade for flow improved from ‘moderate’ in 2018-19 to ‘good’ in 2019-20. Flows in the Mulgrave Basin retained similar characteristics to modelled predevelopment flows in their capacity to support the key ecological assets of water holes, low flow spawning fish, riffle habitats and fisheries production.

The flow indicator provides scores for each flow assessment site and this can be used to assess flows at more local scales. More information on the results of the flow indicator is available in the results technical report.

Fish

The freshwater fish assessment for 2019-20 was conducted at 13 sites in the Mulgrave Basin.

The index improved from ‘good’ with a score of 76 in 2017-18 to ‘very good’ with a score of 84. A total of 38 species were caught including three species introduced into Australia, the guppy (Poecilia reticulata) the platy (Xiphophorus maculatus) and spotted tilapia (Pelmatolapia mariae).

The indicator for species diversity scored 79 and was graded ‘good’, meaning native species expected to occur were frequently caught.

The indicator for introduced fish species scored 89 and was graded ’very good’ due to low numbers of alien fish caught (species introduced into Australia) and no translocated fish caught (Australian species that do not naturally occur in the waterway).

More information on the fish survey including the sites and the fish species list is available in the methods and results technical reports.

See detailed report for further information on the methods used to produce the scores and grades and detailed results