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

Water quality

Water quality in the Mulgrave basin remained ‘good’ in 2016-17 with the score unchanged at 63.

The dissolved inorganic nitrogen (DIN) indicator scored the lowest (37) and was graded ‘poor’, with most monthly median concentrations not meeting guideline values. Filterable reactive phosphorus (FRP) and TSS (the indicator for sediment) were both ‘good’, with monthly median concentrations often meeting guideline values.

Using the ms-PAF method, pesticides received a low risk category with 95.8 % of species protected in the waterways of the Mulgrave basin. This resulted in a pesticide grade of ‘good’ for the basin.

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, riparian extent (both to be updated for 2017-18), invasive weeds (to be updated for 2019-20), habitat modification (impoundment length (to be updated for 2018-19) and fish barrier indicators (in development)). The flow indicator for the Mulgrave basin (reported annually) was introduced in 2016-17.

Changes in the habitat and hydrology index scores for the last three years are the result of the addition of indicators rather than changes in the indicator scores themselves.

The habitat and hydrology index remained ‘good’ in 2016-17 with the score decreasing from 66 to 65 due to the addition of the flow indicator.

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 are 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 34 (‘poor’) with 36.2% loss of palustrine (freshwater) wetlands from pre-clear to 2013. These results reflect a high level of historic loss due to development. Wetland losses in future reporting periods will be minimal.

The invasive weeds indicator scored 52 (‘moderate’) in the Mulgrave basin 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 was graded as ‘good’ for flows during 2016-17. Flows in the Mulgrave basin were not substantially different to 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.


The fish index is currently under development and will be available in future report cards.

Further information on the methods used to produce the scores and grades and detailed results can be found here.