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’ and the score decreased from 71 in the previous year to 68.

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 in 2018-19 for the Mulgrave Basin remained ‘good’ with the score of 66 unchanged from 2017-18.

The sediment score, comprised of the total suspended solids (TSS) indicator, scored 78 (‘good’) decreasing from ‘very good’ the previous year. Monthly median concentrations of TSS often met guideline values.

The grade for nutrients improved from ‘moderate’ in the previous year to ‘good’ and scored 72. The dissolved inorganic nitrogen (DIN) indicator improved from ‘poor’ to ‘moderate’, with some monthly median concentrations meeting guideline values. Filterable reactive phosphorus (FRP) remained ‘good’, with monthly median concentrations often meeting guideline values.

Pesticides for the Mulgrave Basin improved from “moderate” in the previous year to ‘good’ and scored 69, equating to over 96 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 the flow indicator category (updated annually) and four longer-term indicator categories that are updated every four years: wetland extent, (updated for 2017-18), riparian extent (to be updated for 2019-20), instream habitat modification consisting of impoundment length (updated for 2018-19) and fish barrier indicators (in development), and invasive weeds (to be updated for 2019-20).

The annual scores for habitat and hydrology index for 2015-16 and 2016-17 represented changes due to the addition of the invasive weeds and flow indicators, respectively, and not changes in the indicator scores themselves. The index score for 2018-19 includes the third year of reporting for the flow indicator. The Mulgrave Basin habitat and hydrology index scored 63 for 2018-19 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 are impounded by artificial structures. There was no change in impoundment length score for the 2018-19 assessment.

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 52 (‘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 declined from ‘very good’ in 2017-18 to ‘moderate’. Flows in the Mulgrave Basin were altered from modelled pre-development flows, particularly the low flows during the dry season. However, the combination of very low rainfall from August to November and high to very high rainfall in wet season may have resulted in the flow scoring tool overestimating dry season flows.

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 for basins was introduced for the first time in 2017-18 and included both the Mulgrave and Russell basins.

For the Mulgrave Basin the fish index was graded ‘good’ and scored 76. Both the native species richness and pest fish indicators scored 76. A total of 35 native fish species and 4 pest fish species were sampled in the Mulgrave Basin. Of the 16 sites used to calculate the fish index in the Mulgrave Basin two were ‘very poor’, one was ‘moderate’ and the remainder were ‘good’ or ‘very good’.

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

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