Russell-Mulgrave Estuary: Wet Tropics Waterways

The area of the Russell-Mulgrave 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 1.5 km north and 2 km south of the river mouth along the coastline and extends approximately 5.2 km inland. The area includes the confluence of the Russell River and Mulgrave River and several coastal tributaries that drain into the main river channels.

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 Russell-Mulgrave estuary area is dominated by grazing in native vegetation in the downstream reaches with cropping more dominant upstream of the confluence on both the Russell and Mulgrave rivers, with some residential development. The localities of Deeral and Bellenden Ker are just west of the estuary area.

The Russell-Mulgrave estuary grade has remained ‘good’ in 2021-22 with the score decreasing from 73 in the previous year to 69.

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 remained ‘good’ with the score decreasing from 79 to 72.

DIN (dissolved inorganic nitrogen) remained the lowest scoring indicator and declined from ‘moderate’ to ‘poor’ with monthly median values often not meeting guideline values.

Dissolved oxygen (low) remained ‘good’, with monthly median values often meeting guideline values.

FRP (filterable reactive phosphorus) declined from ‘very good’ to ‘moderate’ with monthly median values sometimes meeting guideline values. Chlorophyll a and turbidity remained ‘very good’, with monthly median values mostly meeting guideline values.

Pesticides remained ‘good’ with the score decreasing from 75 to 73, equating to over 97 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. The results, excluding pesticides, are derived from two monitoring sites in the channel at the confluence of the rivers. Pesticide results were based upon monitoring at the same Great Barrier Reef Catchment Loads Monitoring Program sites on the Mulgrave and Russell rivers as used for the basins.

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 and shoreline habitat updated in 2021-22), riparian extent (updated in 2021-22) and fish barriers (date of next update to be confirmed). The habitat and hydrology index remained ‘good’ and the score decreased from 69 in 2020-21 to 67.

Mangrove and saltmarsh scored 84 (‘very good’). Shoreline mangrove habitat scored 70 (‘good’) whilst and mangrove and saltmarsh extent scored 97 (‘very good’) with 0.5% loss from pre-clear to 2019. There was no change in extent between the previous assessment for 2017 and the updated assessment for 2019. These results reflect a low level of development within the estuary area.

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. Riparian assessment therefore includes areas that have been cleared for agriculture and other developments. Riparian extent scored 24 (‘poor’) with 47% loss from pre-clear to 2019. There was no change in extent between the previous assessment for 2017 and the updated assessment for 2019.

Estuary fish barriers for the Russell-Mulgrave estuary scored 81 (‘very good’) and were graded ‘very good’ for barrier density, ‘good’ for percentage of stream length to the first barrier, with an absence of low passability barriers (graded ‘very good’).

For flows, the Russell-Mulgrave estuary remained ‘very good’ with the score decreasing from 84 to 81. Flows to the Russell-Mulgrave estuary were similar to modelled predevelopment flows in their capacity to support key ecological values including water holes, low flow spawning fish, riffle habitats and fisheries production.

More information on the results of the flow indicator is available in the results technical report.